Triple-Chocolate Mousse Cake

Ingredients

• Bottom Layer:
• 6 tablespoons (¾ stick) butter, cut into 6 pieces
• 7 ounces bittersweet chocolate, chopped fine
• ¾ teaspoon instant espresso powder
• 1 ½ teaspoons vanilla
• 4 large eggs, separated
• Pinch of salt
• 1/3 cup packed light brown sugar, crumbled with fingers to remove lumps
• Middle Layer:
• 2 tablespoons cocoa powder
• 5 tablespoons hot water
• 7 ounces bittersweet chocolate, chopped fine
• 1 ½ cups cold heavy cream
• 1 tablespoon granulated sugar
• 1/8 teaspoon salt
• Top Layer:
• ¾ teaspoon powdered gelatin
• 1 tablespoon water
• 6 ounces white chocolate chips
• 1 ½ cups cold heavy cream

Directions

1. Bottom layer: Adjust oven rack to middle position and heat oven to 325 degrees. Butter the bottom and sides of a 9 ½ inch springform pan. Melt butter, chocolate, and espresso powder in a large heatproof bowl set over a saucepan filled with 1 inch of barely simmering water, stirring occasionally until smooth. Remove from heat and cool slightly, about 5 minutes. Whisk in vanilla and egg yolks; set aside.
2. With mixer and whisk attachment, beat egg whites and salt at medium speed until frothy, about 30 seconds. Add half of the brown sugar and beat until combined, about 15 seconds. Add remaining brown sugar and beat at high speed until soft peaks form when the whisk is lifted, about 1 minute longer, scraping down the sides halfway through. Using a whisk, fold one third of the beaten egg whites into the chocolate mixture to lighten. Using a rubber spatula, fold in the remaining egg whites until no white streaks remain. Carefully transfer batter to the prepared springform pan, gently smoothing the top with an offset spatula.
3. Bake until the cake has risen, is firm around the edges, and the center has just set but is still soft, 13-18 minutes. Transfer cake to a wire rack to cool completely, about 1 hour. (Cake will collapse as it cools.) Do not remove the cake from the pan.
4. Middle layer: Combine cocoa powder and hot water in a small bowl; set aside. Melt chocolate in a large heatproof bowl set over a saucepan filled with 1 inch of barely simmering water, stirring occasionally until smooth. Remove from heat and cool slightly, 2-5 minutes.
5. In a clean mixer bowl fitted with the whisk attachment, whip cream, sugar, and salt at medium speed until the mixture begins to thicken, about 30 seconds. Increase speed to high and whip until soft peaks form when the whisk is lifted, 15-60 seconds.
6. Whisk the cocoa powder mixture into the melted chocolate until smooth. Using a whisk, fold 1/3 of the whipped cream into the chocolate mixture to lighten. Using a rubber spatula, fold in the remaining whipped cream until no white streaks remain. Spoon the mousse into the springform pan over the cooled cake and gently tap the pan on the counter 3 times to remove any large air bubbles; gently smooth the top with an offset spatula. Wipe the inside edge of the pan with a damp cloth to remove any drips. Refrigerate the cake at least 15 minutes while preparing the top layer.
7. Top layer: In a small bowl, sprinkle gelatin over water; let stand at least 5 minutes. Place white chocolate in a medium bowl. Bring ½ cup cream to a simmer over medium-high heat. Remove from heat; add the gelatin mixture and stir until fully dissolved. Pour the cream mixture over the white chocolate and whisk until the chocolate is melted and the mixture is smooth, about 30 seconds. Cool to room temperature, stirring occasionally, 5-8 minutes (mixture will thicken slightly).
8. With mixer and whisk attachment, whip the remaining cup of cream at medium speed until it begins to thicken, about 30 seconds. Increase speed to high and whip until soft peaks form when the whisk is lifted, 15-60 seconds. Using a whisk, fold one third of the whipped cream into the white chocolate mixture to lighten. Using a rubber spatula, fold the remaining whipped cream into the white chocolate mixture until no white streaks remain. Spoon the white chocolate mousse into the pan over the middle layer. Smooth the top with an offset spatula. Return the cake to the refrigerator and chill until set, at least 2 ½ hours.
9. To serve: Run a thin knife between the cake and the side of the springform pan; remove the side of the pan. Cut into slices and serve.