Banana-Hazelnut Praline: Line a baking sheet with parchment paper. Set aside.
In a large heavy skillet over medium heat, toast the hazelnuts until fragrant, about 4 minutes. Immediately remove from the pan onto a dish; wipe the pan clean with a dry towel.
Place the water, granulated sugar and corn syrup in the pan, stir to combine, and return to the stove. Increase the heat to medium-high. Cook without stirring until the sugar begins to turn a golden caramel.
Immediately turn the heat to low and add the hazelnuts, banana chips and salt, and stir to coat.
Spread the praline on the parchment-lined baking sheet to cool. When cool, break the praline into pieces with your hands, or pulse into small pieces in a food processor.
Preheat the oven to 350 degrees F. Line cupcake pans with 24 baking cups.
In a stand mixer with the paddle attachment, combine the bananas, granulated sugar and brown sugar and mix on medium-low until well-combined, about 1 minute.
With the mixer running, slowly pour in the oil and vanilla. Continue mixing until combined. Stop the mixer, remove the bowl and paddle, and use the paddle to scrape the inside of the bowl, making sure everything is fully incorporated. Sift together the flour, baking powder, baking soda, salt, cinnamon and nutmeg and add to the batter. Reattach the bowl and paddle to the mixer and mix on medium until just combined, then add the eggs and continue mixing until the batter is smooth, about 10 to 20 seconds. Remove the bowl and paddle from the mixer and, once again, use the paddle to scrape the bottom and sides of the bowl, ensuring that everything is well-mixed.
Scoop the batter into the prepared baking cups, filling them 2/3 of the way.
Bake in the middle of the oven for 20 to 25 minutes, rotating the pan halfway through. The cupcakes are done once the centers spring back when you touch them.
Remove the cupcakes from the oven and let cool completely while you make the buttercream.
For French Buttercream (recommended): Prepare the recipe as directed below. Once the butter is added, add the Nutella and cocoa powder and mix on high to combine.
Fill a pastry bag fitted with a fluted tip with the Nutella buttercream and pipe onto each banana cupcake.
Place pieces of banana-hazelnut praline on the top of each cupcake.
Skip the praline. If you still want a crunchy top, you can sprinkle some crushed banana chips, roasted hazelnuts or a mixture of the two on top.
This is it: the star of the show, the main event, the most important part of our cupcakes - buttercream! As this is French buttercream, this is tres, tres serious. You will pay attention, follow along and execute these directions! C'est serieux! Now let's do this!
In a heavy nonreactive saucepan, add water, sugar, corn syrup and cream of tartar. The last two help keep the sugar from crystallizing. (Candy thermometer is nonnegotiable!)
Put the pan on high heat. It's going to be there for a while. Be patient and keep your eye on it. Don't go walking away and watch TV or something.
Put yolks and egg in a stand mixer with the whisk attachment and turn to high. Just let it go! Eggs will triple in volume and go to the "ribbon stage." You can't overwhip!
Wait on the sugar - you're looking for 235 degrees F, aka "soft-ball stage." When it happens, be ready to move quickly. Turn off the mixer and add xanthan gum, then turn back on to medium speed. Remove the thermometer from hot sugar. Lift pan with two hands.
Rest the lip of the saucepan on the edge of the mixer bowl. Slowly tilt and pour sugar in a sloooow steady stream down the side of the bowl. Don't go too fast! If you do there will be chunks of scrambled eggs in your buttercream.
Once sugar is all in, turn the mixer to high. Beat til cool. Gauge this by putting the inside of your wrist on the outside of the bowl. It's more accurate than your hands.
Switch out the whisk for the paddle. Next we're adding the butter. It's too heavy for the whisk and you'll end up breaking your stand mixer if you stay with the whisk.
Start cutting the butter into thin pieces - you could shave it with a cheese slicer if you'd like. Add butter piece by piece - pain in the derriere, yes, but we're making an emulsion.
See, if you dump all the butter in at once, the butter and eggs will never combine properly and you'll have a "broken" buttercream. You'll be able to identify this easily - it'll be a chunky, watery, hot mess.
If your buttercream does break, you can fix it! Turn to medium-high, then add a little more butter, piece by piece, til fixed. Or try adding a little guar gum! This is very strong, so only add a pinch and beat for a minute, then check.
Once your butter is added, turn the mixer to medium-high to add some air - 10 or 20 seconds at most. Quelle magnifique! It should be fluffy and make you want to eat it with your fingers.
Once you have your base, there are so many ways to flavor it! Coffee powder! Vanilla beans! Dutch cocoa! Peanut butter! Caramel! Ham! (OK; maybe not ham. Party pooper.)
Congratulations! You did it! You made French buttercream!
From Robicelli's: A Love Story, with Cupcakes by Allison and Matt Robicelli. Reprinted by arrangement with Viking Studio, a member of Penguin Group (USA) LLC, a Penguin Random House Company. Copyright (c) Allison and Matt Robicelli, 2013