- 3/4 cup cocoa powder
- 3/4 cup water
- 4 ounces unsweetened chocolate, chopped or broken into 1-inch pieces
- 2 cups all-purpose unbleached flour
- 1 1/2 teaspoons baking soda
- 1/4 teaspoon kosher salt
- 12 tablespoons (1 1/2 sticks) unsalted butter, softened
- 2 cups granulated sugar
- 5 large eggs
- 1 cup whole milk
- 10 ounces bittersweet chocolate, chopped (no bigger than 1/2-inch pieces)
- 1 cup heavy cream
- 2/3 cup granulated sugar
- 12 tablespoons (1 1/2 sticks) unsalted butter, softened and cut into 1-inch pieces
For the cake:
- Preheat the oven to 350°F. Grease the bottoms and sides of the cake pans with cooking spray or butter and coat evenly with cocoa powder, tapping out the excess.
- In a small bowl, stir the 3/4 cup cocoa powder with the water to make a paste and set aside.
- Melt the chocolate by putting it in a heat-proof bowl set over a saucepan of simmering water, making sure the bowl does not touch the water. (You can use a double boiler if you have one.) Stir and scrape the side of the bowl occasionally with the rubber spatula until the chocolate is smooth and evenly melted. Cool to room temperature.
- Over a bowl or piece of parchment, sift the flour and baking soda together in a sifter or in a fine strainer by gently tapping your hand against the edge. Add the salt. Set aside.
- Using the stand mixer, or a handheld mixer and medium bowl, on medium speed, beat the butter and sugar together until smooth. Scrape down the side of the bowl with the spatula and mix in the eggs, 1 at a time, until combined. On low speed, add the cocoa paste. Add the melted chocolate and mix until thoroughly combined.
- On low speed, add half of the sifted flour, mixing thoroughly, then half the milk. Repeat with the remaining flour and then the milk, mixing well.
- Divide the batter between the pans and, using the spatula, spread it out. Bake the cakes on the middle rack of the oven until a bamboo skewer inserted in the middle comes out clean, about 20 minutes. If you put the cake pans on 2 different racks in the oven,
- switch the pans halfway through baking so they’ll bake evenly.
- Cool the cakes in their pans. Run a small knife around the inside edges. Place a plate on top of each cake and invert the cake and plate. Remove the pans. Cool the cakes completely.
For the frosting:
- Put the chopped chocolate in a medium bowl. Heat the cream with the sugar in a small saucepan over medium-high heat until small bubbles form at the edge, stirring to dissolve the sugar.
- Pour the warm cream over the chopped chocolate. Shake the bowl a little to submerge all the chocolate pieces. Then cover the bowl for several minutes. (You can cover the bowl with anything that’s handy, like a pan lid.) Whisk until smooth. Let sit for 5 minutes to cool slightly.
- Whisk the butter into the chocolate, adding only 1 or 2 pieces at a time and whisking until they’re blended before adding the next. Once all the butter is added, give the mixture a final whisk to smooth it out. Let sit at room temperature until it seems spreadable. This step may take up to an hour, but you can prepare the frosting up to 6 hours ahead of time. If you try to rush it by putting it in the refrigerator, stir every 10 minutes and keep a close eye on it so it doesn’t get hard.
- To assemble the cake, put one of the cake layers bottom-side up on the cake plate or platter. Using the narrow metal spatula, spread with about 1 cup of the frosting. With the wide spatula, place the other layer (also bottom-side up) on top of the frosting. Spread the frosting over the side and then the top of the cake.