To make the eclairs: Center a rack in the oven and preheat the oven to 425 degrees F. Line a baking sheet with parchment paper or a silicone baking mat.
There are several ways to shape your bubble eclairs. Usually I portion the dough with a small cookie scoop (one with a capacity of 2 teaspoons), but you can pipe the dough from a pastry bag with a plain or open star tip, which makes squiggles and swirls that look lovely when baked. You might want to make it simple and just use a zipper-lock plastic bag as a piping bag.
No matter what you use, you want to scoop or pipe 3 balls of dough as close to one another as you can get them for each eclair. If you're scooping, scoop 1 ball, then crowd it with the second-giving it a little nudge-and then scoop out the last bubble, snuggling it up to the second ball of dough. When you've got 3 bubbles in a row, move on to the next eclair, making sure to leave at least 2 inches of puff-and-grow space between the clusters. If you're piping, the principle is the same.
Now you have to make some decisions about the topping. If you're going to top the bubbles with disks of crackle, there's no need to give them an egg wash. Just lay a circle of the Crackle Top Dough on each puff of dough.
If you'd like to top the eclairs with Streusel or pearl sugar, then it's nice to give each puff a swipe of egg wash. Make an effort to keep the egg wash on the top of the puffs and not let it drip down the sides; it's not fatal, but it does hamper the even rise of the pate a choux. If you're using Streusel, break up the pieces with your fingertips and use a smidge on each puff-put it on and press it down very, very lightly. If you're using pearl sugar, go to town-it's nice to have that crunch.
Slide the baking sheet into the oven and immediately turn the oven temperature down to 375 degrees F. Bake the eclairs for 20 minutes without opening the oven door, then rotate the pan and continue to bake the pastries for another 10 to 15 minutes, or until they are golden on top and bottom and firm to the touch. Transfer the eclairs to a rack to cool to room temperature.
To fill the eclairs: There are two traditional ways to fill eclairs, both good and both fun. One is to fill a pastry bag with whatever you've chosen as a filling, use a medium plain pastry tip to poke a hole in the eclair-either in the bottom or in one side-nuzzle the tip into the hole and squeeze in enough filling to come to the edges of the hole.
The other way is to slice the eclairs horizontally, either in half or by cutting off the top one third. If you do this, you can scoop out the (inevitable) eggy innards. Some people love the custard interior, some don't, so know thyself (and thine audience). Pipe, spoon or scoop the filling into the bottom of the eclairs and cap with the tops.
No matter how you've filled the pastries, they should be served immediately or stowed in the refrigerator until needed.
If you haven't sugared or crackled your eclairs, you might want to give them a last-minute dusting of confectioners' sugar.
Serving: There's no law that says you can't pass these around and let everyone eat them out of hand, but I think of them as knife-and-fork pastries. If you serve them with cutlery you might want to serve them with bittersweet chocolate sauce or hot or cold salted caramel sauce.
Bring the milk, water, butter, sugar and salt to a rapid boil in a medium heavy-bottomed saucepan over high heat. Add the flour all at once, lower the heat to medium-low and quickly start stirring energetically with a wooden spoon or sturdy heatproof spatula. The dough will come together and a light crust will form on the bottom of the pan. Keep stirring vigorously for another minute or two to dry the dough. The dough should be very smooth.
Turn the dough out into the bowl of a stand mixer fitted with the paddle attachment, or into a large bowl in which you can use a hand mixer or a wooden spoon. Let the dough sit for 3 minutes, then add the eggs one by one, beating until the dough is thick and shiny. Make sure that each egg is fully incorporated before you add the next, and don't be concerned if the dough falls apart; by the time the last egg goes in, it will have come together again. The dough should be used immediately.
Remove the butter from the refrigerator 10 minutes before you're ready to make the dough.
After 10 minutes, cut the butter into small cubes and toss it and the brown sugar and salt into a food processor. Process until almost blended. Add the flour and pulse until the dough forms moist curds. Pulse in the vanilla.
Scrape the dough onto a piece of parchment or wax paper and shape it into a disk. Cover with another piece of parchment or wax paper and roll the dough out so that it's about 1/16th inch thick. Slide the dough, still between the paper, onto a cutting board and freeze it for at least 2 hours. (The dough can be frozen for up to 2 months, in which case it should be wrapped airtight.)
Put the flour, sugar and salt in a medium bowl and mix everything together by running the ingredients through your fingers. Drop in the butter and squeeze and rub the cubes into the mixture until you have a sandy mix. If you press a little streusel together, you'll get lumps, which is just what you want. Drizzle the vanilla over the streusel and toss, squeeze and rub to distribute it. Cover the bowl and chill the streusel until needed. (You can make the streusel up to 1 week ahead; pack it in an airtight container and keep it refrigerated.)
If using a vanilla bean, cut it in half lengthwise and use the tip of the knife to scrape out the pulpy seeds. Bring the milk and vanilla bean pulp and pod to a boil in a medium saucepan. (If you're using extract, you'll add it later.) Cover the pan, turn off the heat and let infuse for at least 10 minutes, or for up to 1 hour.
Return the milk to a boil if you've used a vanilla bean; bring it to a boil if you're using extract.
Whisk the yolks, sugar and cornstarch together in a medium heatproof bowl until blended. Whisking constantly, drizzle in one quarter of the hot milk to temper, or warm, the yolks. Still whisking, add the remainder of the liquid in a steady stream. Remove the pod (if you have it), pour the mixture into the saucepan and, whisking vigorously and constantly, bring to a boil over medium heat. Keep at a boil-never stop whisking-for 1 to 2 minutes, then press the cream through a sieve into a clean bowl. Stir in the vanilla extract, if you're using it.
Leave the pastry cream on the counter for 10 minutes, then whisk in the butter until it is fully incorporated and the pastry cream is smooth and silky.
Either press a piece of plastic film against the surface of the cream and refrigerate it until it's thoroughly chilled, at least 2 hours, or fill a large bowl with ice cubes and cold water, set the bowl of pastry cream in it and leave the cream there, stirring occasionally, until it's cold. Use now, or cover and refrigerate until needed.
Storing: The pastry cream can be kept tightly covered in the refrigerator for up to 3 days. Whisk it well to loosen it before using.
Working in the bowl of a stand mixer fitted with the whisk attachment, or in a large bowl with a hand mixer or a balloon whip, beat the cream and 1 to 2 tablespoons sugar together just until the cream starts to thicken. Taste and add more sugar, if you'd like, then continue to beat until you get the thickness you want. I use my stand mixer and start off slow, then increase the speed and when the cream is almost whipped the way I want it, I finish it by hand with a whisk. Hand finishing reduces the risk of overbeating and turning your cream into sweetened butter. When the cream is just right, stir in the vanilla extract, if you're using it.
Storing: Whipped cream is best served soon after it's made, but it will keep, covered, in the refrigerator for a few hours. Give it a go-round with a whisk just before serving. To keep the cream longer- or to get a thicker cream- line a strainer with a piece of moistened cheesecloth, place the strainer over a bowl and put the cream in the strainer. Wrap the whole setup in plastic film and refrigerate it for up to 24 hours.
Working in the bowl of a stand mixer fitted with the whisk attachment, or in a large bowl with a heavy-duty hand mixer, beat the cream until it holds medium-firm peaks. If you're working in a stand mixer, transfer the cream to another bowl and rinse and dry the mixer bowl and whisk. Cover the whipped cream and refrigerate.
Put the eggs and yolks in the mixer bowl and fit the machine with the whisk attachment, or work in another large bowl with the hand mixer. Beat the eggs at low speed just to break them up. Set aside while you prepare the chocolate and syrup.
Melt the chocolate in a microwave oven or in a heatproof bowl set over a saucepan of simmering water, making sure that the heat isn't too high and that the chocolate remains thick and shiny. If necessary, scrape the chocolate into a bowl that's large enough to hold all the ingredients for the mousse; set aside. The chocolate must cool (114 degrees F is the ideal temperature) so that when you fold in the cold whipped cream, it doesn't seize and clump.
Bring the sugar and water to a boil in a small heavy-bottomed saucepan. Swirl the pan to mix the ingredients as they start to heat, and if some of the sugar spatters on the sides of the pan, wash it down with a silicone pastry brush dipped in cold water. Cook the sugar over high heat until it reaches 257 degrees F on a candy or instant-read thermometer, about 8 minutes (or maybe a little more, depending on the dimensions of your pan and the heat you have under it).
Back to the eggs: Beat the eggs on the lowest speed, just to get them moving again, then very slowly pour in the hot sugar syrup in a thin stream. Try to pour the syrup between the side of the bowl and the whirring whisk. Please be careful-this is soooooooo hot and sticky! No matter how careful you are, you'll get spatters and you should just leave them-trying to scrape them into the eggs will only produce lumps. When all the syrup is in the bowl, up the mixer speed to high and beat until the eggs are pale and more than doubled in volume, about 5 minutes. If the eggs have doubled but still feel warm, lower the mixer speed to medium and keep beating until they reach room temperature.
Pull the whipped cream from the refrigerator and give it a couple of rounds with a whisk, just to bring back the medium-firm peaks. Working with a large flexible spatula, fold about one quarter of the whipped cream into the chocolate to lighten it, then gently fold in the rest of the cream. Now, using the spatula and your lightest touch, fold in the whipped eggs. Your voluminous eggs will deflate some, but be gentle and the texture of your mousse will be lovely.
Use the mousse now if you're filling a cake with it, but if you want to pipe it, it's best to cover and chill it for at least 1 hour. And if you are serving it on its own, chill it.
Serving: The mousse can be served elegantly in small cups or bowls, topped with a rosette of unsweetened whipped cream, or family style, as it's often served in French homes.
Storing: The mousse can be covered tightly and kept in the refrigerator for up to 1 day.
Storing: Once the eclairs are filled, they're best served immediately (especially if they're filled with ice cream) or stored in the refrigerator and eaten the same day. However, as with most pastries made from pate a choux, you can freeze the piped-out eclairs and then bake them straight from the freezer. You can also wrap the unfilled baked pastries and freeze them for up to 2 months. Defrost them in the refrigerator, warm them in a 350-degree-F oven for 10 minutes, cool and then fill them.
"Baking Chez Moi" by Dorie Greenspan (c) Houghton Mifflin Harcourt 2014. Provided courtesy of Dorie Greenspan. All rights reserved. Photo by Alan Richardson.