For the Cake
- 21/3 cups cake flour
- 2/3 cup Dutch-process cocoa powder
- 11/2 teaspoons baking powder
- 11/2 teaspoons baking soda
- 1 teaspoon kosher salt
- 1 cup granulated sugar
- 3/4 cup plus 2 tablespoons packed light brown sugar
- 2 large eggs
- 1 cup full fat buttermilk
- 2 teaspoons vanilla extract
- 1/4 cup plus 1 teaspoon oil, such as safflower or canola
- 4 tablespoons unsalted butter
- 1 cup water
- 1/2 teaspoon espresso powder
To Make the Cake
Preheat oven to 350ºF. Prepare two 8” round cake pans by greasing the bottoms and sides with melted or softened butter. Place a parchment circle on the bottom of each cake pan. Set aside.
In a large bowl sift together the cake flour, cocoa powder, baking powder, baking soda, and salt. Mix in the white and light brown sugars. In a medium bowl combine the eggs, buttermilk, and vanilla extract.
In a small saucepot bring the water to a boil. Stir in the oil, butter, and espresso powder. Add the buttermilk mixture to the dry ingredients and using a whisk, stir gently to combine. Add in the hot water mixture in a steady, slow stream.
Divide evenly between the two pans and bake for 30 minutes. Check the pans at this point. If the top of the cake looks set and isn’t liquidy you may rotate the cakes. Otherwise continue to bake at 15 minute intervals, only rotating once the cakes no longer appear jiggly. The cakes are done when the surface of the cake springs back when (lightly!) touched and a ⅛” gap forms between the cake and the sides of the pan., about 35-45 minutes depending on your oven.
Let the cakes rest in the pans for about 30 minutes before releasing them. Wrap in plastic wrap with a cardboard cake circle on the bottom. The cake will keep as is for 2 days at room temperature or three months frozen.
For the Chocolate Mousse Filling
- 3 ounces bittersweet chocolate
- 6 tablespoons unsalted butter
- 2 tablespoons coffee, or water, hot
- 1 teaspoon vanilla extract
- 1/4 cup plus 1 tablespoon granulated sugar, divided
- 2 large eggs, separated
- 1/4 teaspoon kosher salt
- 1/2 cup heavy whipping cream, cold
To Make the Chocolate Mousse Filling
Add the chocolate, butter, hot coffee or water, and vanilla extract to a small bowl set over a saucepot with 2 inches of water. Bring the water to a simmer and stir occasionally until the mixture is completely melted and combined. Remove from the heat and set aside to cool.
Add the egg yolks and 1 tablespoon of granulated sugar to a medium bowl and set over the saucepot with simmering water. Whisk vigorously until the mixture has thickened and is a pale yellow color. Remove from the heat. Slowly add the chocolate mixture to the egg mixture until mostly homogenous. Some streaks are ok. Set aside.
In the bowl of a stand mixer fitted with the whisk attachment add the egg whites. Mix on medium speed with the kosher salt until frothy. Slowly add the 1/4 cup granulated sugar to the whisking egg whites. Once all of the sugar has been added raise the speed to medium high and continue to whisk until soft peaks form. Add the egg whites to the chocolate mixture and fold in gently.
Finally, whip the heavy cream to soft peaks on medium speed. Fold into the chocolate mixture until homogenous. Let the mousse chill completely before using, about 4 hours.
For the Bittersweet Chocolate Swiss Meringue Buttercream
- 1/2 cup egg whites
- 1 cup plus 2 tablespoons granulated sugar
- 4 sticks unsalted butter, at room temperature
- 8 ounces bittersweet chocolate
- 2 teaspoons kosher salt
- 4 tablespoons Dutch-process cocoa powder
- 2 tablespoons black cocoa powder
- 2 teaspoons vanilla extract
To Make the Bittersweet Chocolate Swiss Meringue Buttercream
Prepare a double boiler by filling a high-sided saucepan with 2 inches of water. Weigh the chocolate into a heat safe bowl and set the bowl over the saucepan. Turn the heat to medium low. Let the chocolate melt completely, stirring as little as possible. One the chocolate has melted add in the salt and dutch-processed cocoa powder. Stir gently to combine until the cocoa powder has dissolved. Don’t worry if the mixture looks curdled. Set aside.
Meanwhile, heat the egg whites and sugar over a bain marie to 155°F, stirring constantly with a whisk. The mixture will feel smooth, not gritty, when tested with your finger. Place the bowl on your stand mixer and whip at high speed until cool and thick.
Quickly add the butter to the mixer, whipping on medium-high speed until all of the butter has been incorporated. Continue to beat the mixture until it is emulsified and creamy. Switch out the whisk attachment for the paddle attachment and continue to whip on high speed for one more minute.
In a small bowl add the black cocoa powder and vanilla extract. Add hot water (you’ll have some to use from your double boiler set up!) until a thick paste forms. Set aside.
Add the cooled chocolate mixture in thirds, whipping the buttercream on high speed for 20 seconds between each addition. Add in the black cocoa powder paste and whip on high speed for 20 seconds. Scrape down the sides and whip again if necessary.
The buttercream can be left at room temperature for up to 5 days, stored in the refrigerator for up to 2 weeks or frozen for 3 months. If refrigerated or frozen let the buttercream come to room temperature and whip on high speed until smooth and glossy before using.
To Assemble the Cake
Use a rotating cake stand to split each cake in half. You will now have 4 layers. Use one of the “bottom” layers for the first layer of the cake and reserve the second “bottom” layer for the top.
Fill a piping bag fitted with a large round tip with about a quarter of the buttercream. Pipe a ring around the border of your cake. This will serve as a dam to keep the mousse filling inside each layer. Use a small offset spatula to smooth a layer of mousse inside the dam. Fit the next cake layer on top and pat gently to adhere. Continue to fill the next two layers as before.
When you are finished with filling the third layer, take your second “bottom” layer and carefully invert it onto the exposed layer of filled cake. You will now have a completely flat top.
Create a crumb coat by adding a large dollop of buttercream to the top of the cake. Smooth the top until some of the crumb is exposed. Use the frosting that has fallen down the sides of the cake to finish the crumb coat, using whatever buttercream you need to make sure the sides are evenly covered. Use a bench scraper or cake comb to smooth the sides. Chill for 30 minutes in the refrigerator before applying the final coat of buttercream. Apply the final coat as desired.