6 Tbsp (3/4 stick) unsalted butter, cut into 6 pieces
7 oz bittersweet chocolate, chopped fine
¾ tsp instant espresso powder
4 large eggs, room tempature and separated
1½ tsp vanilla extract
Pinch table salt
⅓ packed (2⅓ oz) light brown sugar, crumbled to remove lumps
Middle layer-
5 Tbsp hot water
2 Tbsp cocoa powder, preferably Dutch-processed
7 oz bittersweet chocolate, chopped fine
1½ cups heavy cream, chilled
1 Tbsp granulated sugar
¼ tsp table salt
Top layer-
¾ tsp powdered gelatin
1 Tbsp water
6 oz white chocolate, chopped fine
1½ cups heavy cream chilled
Shaved chocolate or cocoa powder for serving (optional)
Instructions
For base layer
Adjust oven rack to middle position and heat oven to 325 degrees. Grease the bottom and sides of a 9-inch wide round 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. Stir occasionally until smooth. Remove from heat and allow mixture to cool slightly, about 5 minutes. Whisk in the egg yolks and vanilla; set aside.
In a stand mixer fitted with a 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 the remaining brown sugar and continue to beat on high speed until stiff peaks form, about 1 minute more. Using a whisk, mix one-third of the beaten egg whites to the chocolate mixture to lighten it. With a rubber spatula, fold in the remaining egg whites gently until no streaks remain. Pour the batter into the prepared springform pan and smooth the top.
Bake until the cake has risen, is firm around the edges, and the center has just set but is still soft (center will spring back when pressed gently with finger), 13 to 18 minutes. Transfer cake to a wire rack to cool completely, about 1 hour. The cake will collapse when it cools. Do not remove cake from pan.
For middle layer
Combine hot water and cocoa powder in a small bowl; set aside. In a large heatproof bowl over simmering water, melt chocolate, stirring occasionally until smooth. Remove from the heat and cool slightly 2 to 5 minutes.
In the clean bowl of a stand mixer fitted with the whisk attachment, whip the cream, sugar and salt on medium speed until the mixture begins to thicken, about 30 seconds. Increase the speed to high and whip until soft peaks form, 30-60 seconds. Whisk in the cocoa powder mixture into melted chocolate until smooth. Using a whisk, mix one-third of the whipped cream to the chocolate mixture to lighten it. Fold in the remaining whipped cream gently with a rubber spatula until no streaks remain. Pour the mousse into the springform pan over the cooled cake and tap gently on the counter 2 to 3 times to remove air bubbles. Gently smooth the top with a spatula. Wipe the inside edge of the pan to remove any drips. Refrigerate for at least 15 minutes while preparing the top layer.
For 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 of the cream to a simmer in a small saucepan over medium-high heat. Remove from heat, stir in gelatin mixture until completely dissolved. Pour cream mixture over white chocolate and whisk until chocolate is smooth. Cool to room temperature, stirring occasionally, 5 to 8 minutes (mixture will thicken slightly).
Using a stand mixer with a whisk attachment, whip the remaining 1 cup of cream at medium speed until it begins to thicken. Increase the speed to high and whip until soft peaks form, 30-60 seconds. Using a whisk, mix one-third of the whipped cream to the white chocolate mixture to lighten it. Fold in the remaining whipped cream gently with a rubber spatula until no 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 2½ hours.
Garnish the top of cake with chocolate curls or dust with cocoa powder.
Recipe by Whit and Wonder at https://www.whitandwonder.com/2013/01/17/triple-chocolate-mousse-cake/