For the bottom layer:
6 T unsalted butter, cut into 6 pieces (plus more for the pan)
7 oz. bittersweet chocolate, finely chopped
3/4 t espresso powder (I omitted this)
1 1/2 t vanilla
4 large eggs, separated
pinch of salt
1/3 c light brown sugar
Middle Mousse Layer:
2T cocoa powder, preferably Dutch processed
5 T hot water
7 oz bittersweet chocolate, finely chopped
1 1/2 c heavy cream
1 T sugar
Top White chocolate mousse layer:
3/4 t powdered gelatin
1 T water
6 oz white chocolate, finely chopped
1 1/2 c heavy cream, divided
chocolate curls or mini chocolate chips
For the bottom layer:
Butter the bottom and sides of a 9-inch spring form pan. Preheat the oven to 325.
In the microwave, combine the butter, chocolate, and espresso powder, if using. Heat in 30 second intervals until smooth. Set aside to cool for five minutes. Whisk in the vanilla and egg yolks.
In a mixer, with the whisk attachments beat the egg whites and salt on medium speed until foamy- about 30 seconds. Add half of the brown sugar in the mixture. Mix for 15 seconds. Add the remaining brown sugar. Beat on high until stiff peaks form, about 1 minute.
Using a whisk, add about 1/3 of the egg white mixture into the cooled chocolate. Gently stir to combine. Add this mixture into the remaining egg white mixture and fold it gently until no streaks remain. Pour into the prepared pan and smooth the top.
Bake 14-18 minutes or until the cake has risen. It should be firm around the edges and the center should be set but still soft. Transfer to a wire rack to cool completely, at least an hour. Do not remove the cake from the pan. If you aren't making the next layers until the next day, cover with plastic wrap and refrigerate.
For the Middle Layer:
Whisk together the cocoa and hot water in a small bowl, set aside. Melt the chocolate in the microwave in 30 second intervals until smooth. Let it cool slightly, 2-5 mintutes.
In a mixer with the whisk attachment, whip the cream, sugar, and salt on medium speed until it starts to thicken, about 30 seconds. Increase the speed to high and whip for one minute, until soft peaks form. Remove the bowl. With a hand whisk, add in the cocoa powder and gently mix well. Add about 1/3 of the whipped cream mixture into the cooled chocolate mixture. Add this into the remaining whipped cream mixture. Gently fold with a rubber spatula until no streaks remain.
Pour this onto the cake layer in the spring form pan. Smooth the top. Refrigerate at least 15 minutes while you prepare the top mousse layer.
For the Top Layer:
Sprinkle the gelatin over the water in a small bowl. Let it stand for five minutes. Put the white chocolate into a bowl. In a small saucepan over medium heat, bring 1/2 c of the cream just to a boil. Remove the pan from the heat. Add the gelatin and water to the hot cream. Stir until dissolved. Pour the hot cream over the white chocolate chips and let it sit for one minute. Stir until smooth. Cool to room temperature, about five minutes, stirring occasionally.
In a stand mixer with the whisk attachment, whip the remaining 1 c of cream at medium speed until it starts to thicken. Increase the speed to high and beat for 60 seconds or until soft peaks form. With a whisk, add 1/3 of the whipped cream mixture to the cooled white chocolate mixture. Gently fold it in. Add this to the remaining whipped cream mixture. Using a spatula, gently fold it in until no streaks remain. Spoon this onto the top of the middle layer. Smooth with an offset spatula.
Chill for at least 2 1/2 hours. Cover with chocolate curls or mini chocolate chips.