Special equipment: a kitchen torch, optional, a stand mixer with paddle and whisk attachments, donut molds, piping bags with number 3 or 4 large round tips and a number 0 round tip, a candy thermometer, Silpat or silicone baking mat
For the whoopie pie: Preheat the oven to 350 degrees F.
Paddle the butter in stand mixer fitted with the paddle attachment at medium speed until soft, light and the consistency of mayonnaise. Use a kitchen torch to gently warm the bowl if necessary.
Add the sugar and paddle at medium speed until light and fluffy, scraping down the sides of the bowl often, about 5 minutes. Add the cocoa powder and continue to paddle at medium speed until completely incorporated. Scrape down the sides of the bowl 3 or 4 times to ensure all the cocoa powder is well mixed in, another 5 minutes.
Add the eggs one at a time and mix at medium speed until well incorporated. Scrape down the bowl and paddle after each addition.
Sift together the flour, baking soda, baking powder, salt and espresso powder in a medium bowl. With the mixture on the lowest speed, alternate adding the dry ingredients and water, beginning and ending with the dry ingredients. Scrape well.
Paddle at top speed until smooth and slightly glossy, 5 to 6 minutes.
Place the cake batter in a piping bag. Spray the donut molds with cooking spray. Pipe a thin layer of cake batter into each donut mold, enough to just cover the bottom, about 1/2 inch thick.
Bake until the cake has set and a tester comes out clean, 5 to 7 minutes. Let cool completely on a rack. Repeast to make 48 donut cakes total.
For the marshmallow: Bloom the sheets of gelatin in some ice water. Combine the 1 1/4 cups sugar, corn syrup and 1/2 cup water in a saucepan and cook on medium-high heat until a candy thermometer reads 238 degrees F.
Meanwhile, when the sugar reaches 218 degrees F, begin to whip the egg whites, salt and lemon juice with a stand mixer fitted with the whisk attachment at medium speed. When egg whites become frothy, gradually add 2 teaspoons granulated sugar. Continue to whip the egg whites to soft peaks. Time this well so egg whites are at soft peaks when the sugar mixture reaches 238 degrees F.
Squeeze the excess water from the bloomed gelatin and add to the hot sugar mixture. Swirl the pot to melt the gelatin; do not stir. Remove from the heat.
With the stand mixer on medium-low speed, slowly pour the hot sugar mixture into the whipping meringue, being careful not to let the sugar hit the whisk and splatter.
When all the hot sugar mixture has been poured in, increase the mixer speed to high. Add a splash of vanilla extract and whip the marshmallow until the bowl is cool to the touch and the marshmallow holds its shape, but is still loose and glossy, about 8 minutes. Place the marshmallow in a piping bag with number 3 or 4 large round tip. Makes about 3 cups
For the hippen tuile: Preheat the oven to 350 degrees F.
Paddle the butter in a stand mixer fitted with the paddle attachment at medium speed until smooth and creamy, about 3 minutes. Add the sugar and paddle at medium speed until light and fluffy, about 5 minutes, scraping down the sides of the bowl often. Slowly add the egg whites and continue to paddle at medium speed until completely combined.
Reduce the speed of the mixer to the lowest setting, add the flour and vanilla extract and mix until combined. The batter will resemble cake batter.
Scoop tuile batter into a pastry bag fitted with a number 0 tip. Chill the batter until firm, about 20 minutes.
Pipe the chilled batter into desired shapes (small rectangles to resemble buckles) on a baking sheet lined with a Silpat or silicone baking mat. The batter will be stiff; loosen it slightly by the kneading pasty bag with your hands. Chill the piped tuile cookies until the batter is firm, about 10 minutes. This is a very important step so that the cookies maintain their shapes.
Bake the tuiles until golden brown, 3 to 4 minutes. Let cool completely before taking the cookies off the baking sheet. The tuiles will crisp as they cool.
To assemble: Pipe a ring of marshmallow onto the bottom of a cake ring. Top with another cake ring, pressing down gently to distribute the marshmallow. Attach a buckle cookie to the marshmallow filling on the side. Repeat with the remaining whoopee pie pieces, marshmallows and tuiles. Enjoy!
Propane gas torches are highly flammable and should be kept away from heat, open flame and prolonged exposure to sunlight. They should be used only in well-ventilated areas. Follow torch manufacturer's instructions for use.