DARK CHOCOLATE MOCHA MOUSSE CAKE

Steps:

• Make the cake: Preheat the oven to 350˚ F. Butter a metal 8-inch square cake pan. Line the bottom and sides of the pan with parchment paper; lightly butter the parchment. Combine the cocoa and espresso powders in a medium bowl. Heat the milk and butter in a small saucepan over medium-low heat until the milk is steaming and the butter melts. Whisk into the cocoa mixture; let cool to room temperature.
• Whisk the flour, baking powder, baking soda and salt in a medium bowl. In a large bowl, beat the eggs and sugar with a mixer on medium-high speed until very pale, thick and pillowy, 5 to 8 minutes. Beat 30 to 45 more seconds to stabilize the egg foam.
• Reduce the mixer speed to low; beat in the cocoa mixture in 2 additions, alternating with the flour mixture in 2 additions. Beat just until the batter is smooth and no dry flour remains.
• Pour the batter into the cake pan and gently tap against the counter to eliminate any air bubbles. Bake until a toothpick inserted into the center comes out clean, about 35 minutes. Let cool 10 minutes in the pan, then turn out the cake onto a rack. Peel off the parchment but let it rest gently on top of the cake while it cools completely. Wipe the cake pan clean and reserve.
• Make the ganache: Combine 1/2 cup heavy cream, 4 ounces chocolate and 1 tablespoon butter in a small saucepan. Gently cook over medium-low heat, stirring, until melted and smooth. Let cool.
• Make the mousse: Combine the remaining 4 tablespoons butter and 4 ounces chocolate in a small microwave-safe bowl. Microwave in 30-second intervals, stirring, until melted and smooth. Whisk in the cocoa powder until smooth. Let cool.
• Meanwhile, combine 1/4 cup heavy cream and the gelatin powder in a small microwave- safe bowl. Let stand until the gelatin absorbs the liquid, about 5 minutes, then microwave until melted, 20 to 30 seconds, whisking. Let cool, stirring occasionally.
• Combine the remaining 1 1/4 cups heavy cream, the sugar and espresso powder in a large bowl. Beat with a mixer on medium speed to dissolve the sugar, about 1 minute. Stream in the cooled gelatin mixture, then increase the speed to medium high and beat until the mixture is thick and stiff peaks form. Fold in the cooled chocolate mixture in 2 or 3 batches until the mousse is well blended.
• Assemble the cake: Line the reserved cake pan with 2 crisscrossed strips of parchment paper (this will help you lift out the finished cake). Carefully slice the cake horizontally into 3 equal layers. Lay the best layer cut-side up in the bottom of the pan and brush with some of the chocolate liqueur, if using. (Use a plate or small cutting board to move the layers; if your top cake layer is domed, use it for the middle of the cake.) Evenly spread half of the mousse on top with an offset spatula, all the way to the edges. Top with a second cake layer. Brush again with chocolate liqueur, then spread half of the ganache evenly on top. Add the final cake layer, cut-side up, and brush with the remaining liqueur. Top with the remaining mousse and ganache. (The assembled cake should fill the pan.)
• Refrigerate the cake until the top is set, about 30 minutes, then wrap the whole cake pan with plastic wrap and refrigerate overnight to set completely.
• Lift the cake out of the pan using the parchment pieces and transfer to a cutting board. Using a hot, dry serrated knife, slice the cake, wiping the knife in between cuts for clean edges.