This Triple Chocolate Mousse Cake is adapted from America's Test Kitchen, and is the best mousse cake we've ever tasted. It's light yet rich, decadent yet not heavy, and absolutely chocolatey. Use the best quality chocolate you can. May be made a day ahead.
Prep Time: 1hr
Cook Time: 30mins
For the Bottom Layer:
6TBsalted butter, sliced into 6 pieces (plus extra for greasing pan)
Adjust oven rack to middle position and heat to 325F. Butter bottom and sides of a 9 to 9.5 inch springform pan (must be at least 3 inches high)
In a glass bowl, melt butter, chocolate, and espresso/coffee powder in microwave at 30 second intervals at 50% power. Repeat and stir until very smooth. (Alternatively, you can place bowl over a saucepan of simmering water.) Cool mixture about 5 minutes. Whisk in the vanilla and egg yolks. Set aside.
In stand mixer fitted with whisk attachment, beat egg whites and pinch of table salt at medium speed until frothy, 30 seconds. Add half of the brown sugar and beat until combined, 15 seconds. Add rest of brown sugar, beat on high until soft peaks form, 1 minute. Scrape sides halfway through. Using a whisk, gently fold in one-third of the beaten egg whites into chocolate mixture. Using rubber spatula, gently fold in the rest of egg whites until no white streaks remain, but don't overdo it.
Carefully transfer batter to greased springform pan, gently smoothing the top. Bake until cake is risen and firm around edges and center has just set but is still soft. Center of cake will spring back after pressing gently with finger, about 14-18 minutes. Transfer to wire rack to cool completely. Do not remove cake from pan.
For the Middle Layer:
Combine cocoa powder and hot water in a small bowl and set aside. In a glass bowl, melt chocolate in microwave set to 50% power in 30 second intervals until smoothly melted. Remove and cool 5 minutes.
In the clean bowl of stand mixer fitted with whisk attachment, whip the cream, sugar, and salt at medium speed until mixture thickens, 30 seconds. Increase to high speed and whip until soft peaks form, 1 minute.
Whisk cocoa powder mixture into melted chocolate until smooth. Gently whisk in one-third of whipped cream into chocolate mixture. Use a rubber spatula to gently fold in rest of whipped cream until no white streaks remain (don't overdo it) Spoon mousse into springform pan over the cooled bottom layer. Gently tap pan on counter to remove large air bubbles, gently smooth top, and wipe inside edge to remove drips. Chill cake while preparing top layer.
For the Top Layer:
In a small bowl, sprinkle gelatin over water and let stand 5 minutes. Place white chocolate in a bowl and set aside. Bring 1/2 cup cream to simmer in a small saucepan. Remove from heat. Add gelatin mixture and stir until fully dissolved. Pour cream mixture over the white chocolate and whisk until fully melted and smooth. Cool to room temp, stirring occasionally.
In clean bowl of stand mixer fitted with whisk attachment, whip remaining cup of cream at medium speed until it thickens, 30 sec. Increase speed to high speed and whip until almost stiff peaks form, 1 minute. Using whisk, gently fold a third of whipped cream into the cooled white chocolate mixture. Using rubber spatula, gently fold in rest of whipped cream just until no white streaks remain. Spoon batter into springform pan over chilled middle layer. Smooth top and chill at least 2.5 hours or overnight.
Serve chilled; carefully run thin knife around edges of pan and gently release sides. Garnish as desired (we recommend melted, drizzled chocolate.) Slice to serve.
This cake can be completed made a day in advance.If you enjoyed this recipe, come back and give it a rating ♡