Triple-Chocolate Mousse Cake

This recipe requires a springform pan with sides that are at least 3 inches high. It is imperative that each layer is made in sequential order. Cool the base completely before topping it with the middle layer. For best results, chill the mixer bowl before whipping the heavy cream. For neater slices, use a cheese wire or dip your knife in hot water before cutting each slice.

1. FOR THE BOTTOM LAYER: Adjust oven rack to middle position and heat oven to 325 degrees. Grease 9-1⁄2-inch springform pan. Melt butter, chocolate, and espresso in large heatproof bowl set over saucepan filled with 1 inch of barely simmering water, stirring occasionally until smooth. Remove from heat and let mixture cool slightly, about 5 minutes. Whisk in egg yolks and vanilla; set aside.
2. Using stand mixer fitted with whisk, whip egg whites, cream of tartar, and salt on medium-low speed until foamy, about 1 minute. Add half of sugar and whip until combined, about 15 seconds. Add remaining sugar, increase speed to high, and whip until soft peaks form, about 1 minute longer, scraping down bowl halfway through. Using whisk, fold one-third of beaten egg whites into chocolate mixture by hand to lighten. Using rubber spatula, fold in remaining egg whites until no white streaks remain. Carefully transfer batter to prepared springform pan and smooth top with rubber spatula.
3. Bake cake until risen, firm around edges, and center has just set but is still soft (center of cake will spring back after pressing gently with finger), 13 to 18 minutes. Transfer cake to wire rack and let cool completely, about 1 hour. (Cake will collapse as it cools.) Do not remove cake from pan.
4. FOR THE MIDDLE LAYER: Combine hot water and cocoa in small bowl; set aside. Melt chocolate in large heatproof bowl set over saucepan filled with 1 inch of barely simmering water, stirring occasionally until smooth. Remove from heat and let cool slightly, 2 to 5 minutes.
5. Using stand mixer fitted with whisk, whip cream, sugar, and salt on medium-low speed until foamy, about 1 minute. Increase speed to high and whip until soft peaks form, 1 to 3 minutes.
6. Whisk cocoa mixture into melted chocolate until smooth. Using whisk, fold one-third of whipped cream into chocolate mixture to lighten. Using rubber spatula, fold in remaining whipped cream until no white streaks remain. Spoon mousse into springform pan over cooled cake, smooth top with rubber spatula, and gently tap pan on counter to release air bubbles. Wipe inside edge of pan with damp cloth to remove any drips. Refrigerate cake for at least 15 minutes while preparing top layer.
7. FOR THE TOP LAYER: In small bowl, sprinkle gelatin over water and let sit for at least 5 minutes. Place white chocolate in medium heatproof bowl. Bring 1⁄2 cup cream to simmer in small saucepan over medium-high heat. Remove from heat, add gelatin mixture, and stir until fully dissolved. Pour cream mixture over white chocolate and let sit, covered, for 5 minutes. Whisk mixture gently until smooth. Let cool to room temperature, stirring occasionally (mixture will thicken slightly).
8. Using stand mixer fitted with whisk, whip remaining 1 cup cream on medium-low speed until foamy, about 1 minute. Increase speed to high and whip until soft peaks form, 1 to 3 minutes. Using whisk, fold one-third of whipped cream into white chocolate mixture to lighten. Using rubber spatula, fold remaining whipped cream into white chocolate mixture until no white streaks remain. Spoon white chocolate mousse into pan over middle layer. Smooth top with rubber spatula. Return cake to refrigerator and chill until set, at least 2-1⁄2 hours. (Cake can be refrigerated for up to 1 day; leave at room temperature for up to 45 minutes before removing from pan.)
9. TO SERVE: Garnish top of cake with shaved chocolate and/or dust with cocoa, if using. Run thin knife between cake and sides of pan; remove sides of pan. Run cleaned knife along outside of cake to smooth. Hold handles of cheese wire and pull wire taut. Using thumbs to apply even pressure, slice down through the cake. Wipe wire clean with dry towel. Make second cut, perpendicular to first. Continue to make cuts around circumference. Serve.

SLICING A SOFT CAKE
To create perfectly smooth slices of soft desserts such as Triple-Chocolate Mousse Cake, the best tool is not a knife. It’s a cheese wire—the minimal surface area produces less drag for cleaner, neater slices. If you don’t have a cheese wire, dental floss will work almost as well.
1. Hold handles of wire and pull wire taut. Using thumbs to apply even pressure, slice down through cake. Wipe wire clean with paper towel.
2. Make second cut, perpendicular to first. Continue to make cuts around circumference.