Devil's Food Cupcakes with Chocolate Mousse Buttercream

A few weeks ago, a friend of mine and I were hanging out at my house. Our plan was to bake something, but we simply couldn't decide what. I pulled out one of my favorite baking books, Sticky, Chewy, Messy, Gooey, and we decided that the recipe had to come from it. (FIY, no, this is not a paid or even sponsored book review, I just think the book is beautiful and has delicious recipes.) When my friend flipped to the page with a glossy picture of a sky-high chocolate mousse cake, we knew that was it.
My friend had never baked before in her life. I mean, never. Not even a chocolate chip cookie. So I figured that a cake that large probably wasn't the greatest first recipe. We decided to make it into cupcakes instead. Little did I know, this recipe would take us several hours! The actual cupcake batter didn't take too long, but we had some issues with the frosting. It never totally emulsified like the recipe says it will - it was delicious, but had tiny bits of butter spread out through it. I think our problem was that we didn't soften the butter enough when we pounded it, so the butter was never beaten as much as it needed to be.
That said, the cake part of the cupcakes was so delicious, the buttery bits in the frosting didn't matter. The cake was super moist, rich, and dark, and nobody minded that the recipe made a huge batch of about 3 dozen cupcakes! Looking back, maybe this wasn't the best first recipe for a new teenage baker, but hey, she won't be easily intimidated by recipes in the future, right? We agreed that these cupcakes are delicious and so worth the time that goes into them.
Enjoy! <3

Devil's Food Cupcakes with Chocolate Mousse Buttercream
(slightly adapted from Sticky, Chewy, Messy, Gooey)

Devil's Food Cupcakes
1/3 cup dutch-processed cocoa powder, sifted
1 teaspoon instant espresso powder
4 ounces semisweet chocolate, finely chopped
1 cup boiling water
2 teaspoons pure vanilla extract
1 cup buttermilk
1/2 cup (1 stick) unsalted butter, at room temperature
1/2 cup vegetable oil
1 cup packed light brown sugar
1 cup granulated sugar
3 large eggs
2 1/4 cups flour
1 teaspoon baking soda
1/2 teaspoon salt

Position a rack in the middle of the oven and preheat to 350 F. Lightly grease 36 cupcake cups, or line with paper liners.
In a large bowl, combine the cocoa powder, espresso powder, and chocolate. Pour in the boiling water and stir until the chocolate is melted and the mixture is smooth. Stir in the vanilla. Let the mixture cool, then stir in the buttermilk.
In another bowl, using an electric mixer set at medium speed, beat the butter and oil together until light and fluffy. Add the sugars and beat until creamy. Beat in the eggs, one at a time, beating well after each addition.
Sift together the flour, baking soda, and salt. Add one third of the flour mixture to the batter and beat at low speed for a few seconds, just until combined. Beat in half of the chocolate-buttermilk mixture, again beating for just a few seconds, just until combined. Scrape down the sides of the bowl with a rubber spatula. Beat in another one third of the remaining flour mixture for no more than a few seconds, just until combined. Add the remaining chocolate-buttermilk mixture, beating just for a few seconds until combined. Finally, fold in the remaining one third of the flour mixture by hand, using a large balloon whisk or rubber spatula, just until no streaks of flour remain.
Divide the batter between the prepared cupcake tins. Bake until a wooden skewer inserted in the center of a cupcake comes out clean, about 20-25 minutes (start checking at 15). Transfer to wire racks and let cool completely.

Chocolate Mousse Buttercream
6 large eggs
1 1/2 cups sugar
2 tablespoons dutch-processed cocoa powder
1 teaspoon pure vanilla extract
1 1/2 pounds (6 sticks) cold unsalted butter
8 to 12 ounces semisweet chocolate, melted

Whisk together the eggs, sugar, and cocoa powder in the metal bowl of a stand mixer.
Fill a large sauté pan or skillet with water and bring to a simmer over medium-high heat. Place the mixing bowl in the simmering water and whisk the egg mixture constantly until the sugar is completely dissolved and the mixture is thick and fluffy and very hot, 3 to 4 minutes. Use an instant-read thermometer to check the temperature of the mixture; it should be anywhere between 120 and 140 F.
Remove the bowl from the simmering water and, using the whisk attachment, beat the eggs at medium-high speed until they are tripled in volume and form soft peaks and the bottom of the bowl is completely cool to the touch, about 10 minutes. Beat in the vanilla and salt.
While the eggs are mixing, unwrap the individual sticks of butter and rewrap them loosely in plastic wrap. Pound the butter 5 or 6 times with a rolling pin, or until the butter is soft and malleable but still cool.
With the mixer speed still on medium-high, add the butter, approximately 2 tablespoons at a time, to the egg mixture, beating in each addition until it is incorporated. Don't start to panic if the buttercream seems too liquidy or looks curdled as you beat in the butter. It will magically emulsify into a smooth, creamy frosting by the time the last bit of butter is mixed in. Hold your breath and keep going!
When the buttercream is smooth and glossy with a subtle brown tint from the cocoa powder, turn off the mixer and, using a rubber spatula, carefully fold in about two thirds of the melted chocolate. Taste the buttercream. You can add as much of the remaining melted chocolate as you like for an evern richer, denser buttercream.

Frost cupcakes with the mousse buttercream. You will probably have some extra frosting depending on how much you use for the cupcakes, but I'm sure you can find something to do with it! If you feel like using it for some vanilla cupcakes, these are delicious.