This is a bit of work to make, but the reaction of your guests will be worth it. Three members of my family immediately announced that they too wanted this cake when their birthdays came around. I made the mousse buttercream with a hand mixer and it took FOREVER, so don't make it unless you have a stand mixer.
- 1⁄3 cup Dutch-processed cocoa powder, sifted
- 1 teaspoon instant espresso powder
- 4 ounces bittersweet chocolate, finely chopped
- 1 cup boiling water
- 2 teaspoons vanilla
- 1 cup buttermilk
- 1⁄2 cup unsalted butter, at room temperature
- 1⁄2 cup canola oil
- 1 cup dark brown sugar, firmly packed
- 1 cup granulated sugar
- 3 large eggs, at room temperature
- 2 1⁄4 cups cake flour
- 1 teaspoon baking soda
- 1⁄2 teaspoon salt
Chocolate Mousse Buttercream
- 6 large eggs
- 1 1⁄2 cups granulated sugar
- 2 tablespoons Dutch-processed cocoa powder
- 1 teaspoon vanilla
- 1 pinch salt
- 3 cups cold unsalted butter
- 10 ounces semisweet chocolate, melted
- 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 and let 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 onto a sheet of waxed paper.
- Add one-third of the flour mixture and beat at low just until combined.
- Beat in half of the chocolate-buttermilk mixture 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 a few seconds.
- Finally, fold in the remaining one-third of the flour mixture by hand, using a large rubber spatula, just until no streaks of flour remain.
- Divide the batter between two 9-inch round cake pans that have been greased and lined with parchment.
- Bake at 350°F 25-30 minutes, or until a wooden skewer inserted in the center comes out clean.
- Transfer to wire racks and let cool 5-10 minutes.
- Invert the cakes onto the racks and peel the parchment paper from the layers.
- Let cool completely before frosting.
- Halve each cake layer horizontally using a long serrated knife (plain dental floss works for me) for a total of four thin cake layers.
- Place one layer cut-side up on a serving plate and top with 1 cup of the mousse buttercream, spreading it evenly.
- Continue stacking the cake, spreading 1 cup of the buttercream in between each layer and placing the layer cut-side down.
- Frost the top and sides of the cake with the remaining buttercream.
- Chocolate Mousse Buttercream:.
- Whisk together the eggs, sugar, and cocoa powder in the metal bowl of a stand mixer.
- Fill a large saute pan or skillet with water and bring to a simmer over mediium-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, fluffy, and very hot, 3-4 minutes.
- Use an instant-read thermometer to check the temperature of the mixture -- it should be anywhere between 120-140°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 (it took 45 minutes with a hand mixer -- don't try it!).
- 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 5-6 times with a rolling pin, until it is soft and malleable but still cool.
- With the mixer speed still on med-high, add the butter 2 T at a time, to the egg mixture, beating in each addition until it is incorporated.
- Don't panic if it seems liquidy or looks curdled; it will magically emulsify.
- When the buttercream is smooth and glossy with a subtle brown tint from the cocoa powder, turn off the mixer and carefully fold in the melted chocolate.