To prepare the bottom layer, heat oven to 325º. Grease a 9- or 9½-inch springform pan.

Combine butter, chocolate and espresso in a large heatproof bowl set over a pan of barely simmering water. Stir occasionally until melted and smooth. Remove from heat and allow to cool slightly, about 5 minutes. Whisk in egg yolks and vanilla and set aside.

Using a stand mixer fitted with the whisk attachment, whip egg whites, cream of tartar and salt on medium-low speed until foamy, about 1 minute. Add half of sugar and continue whipping until combined, about 15 seconds. Add remaining sugar, increase speed to high, and whip until soft peaks form, 1 additional minute, scraping down bowl halfway through.

Using a whisk, fold one-third of the egg whites into the chocolate mixture to lighten it. Fold in remaining egg whites using a rubber spatula. When no white streaks remain, carefully transfer batter to prepared pan. Smooth top with spatula.

Bake cake until it has risen, is firm around the edges and just set in the center (cake center should spring back when gently pressed with finger), 13 to 18 minutes. Transfer to a wire rack to cool completely, about 1 hour. Do not remove cake from pan.

While cake is cooling, prepare middle layer. Combine the hot water and cocoa in a small bowl and set aside. Place chocolate in a large heatproof bowl set over a pan of barely simmering water. Stir occasionally until melted and smooth. Remove from heat and allow to cool slightly, 2 to 5 minutes.

Using a stand mixer fitted with the whisk attachment, whip cream, sugar and salt on medium-low speed until foamy, about 1 minute. Increase speed to high and whip until soft peaks form, 1-3 minutes. (It helps to chill the mixing bowl before whipping the cream.) Whisk cocoa mixture into chocolate.

Using a whisk, fold one-third of the cream into the chocolate mixture to lighten it. Fold in remaining cream using a rubber spatula. When no white streaks remain, spoon mousse onto cooled cake. Smooth top with spatula and gently tap on counter to release any air bubbles. Wipe inside edge of pan with a damp cloth or paper towel to remove any drips. Refrigerate for at least 15 minutes while preparing top layer.

To prepare the top layer, sprinkle gelatin over water and let sit for at least 5 minutes. Place white chocolate in a medium heatproof bowl. Bring ½ cup of cream to a simmer in a small saucepan over medium-high heat. Remove from heat and stir in gelatin mixture until completely dissolved. Pour cream mixture over white chocolate, cover and allow to sit for 5 minutes. Whisk gently until smooth. Let cool to room temperature, stirring occasionally (mixture will thicken slightly during this time).

Using a stand mixer fitted with the whisk attachment, whip remaining cup of 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 a whisk, fold one-third of cream into white chocolate to lighten it. Fold in remaining cream using a rubber spatula. When no white streaks remain, spoon white chocolate mousse into the pan over the middle layer. Smooth top with a rubber spatula. Return to refrigerator and chill until set, at least 2½ hours. Cake can be refrigerated up to 1 day. Allow to sit at room temperature for about 45 minutes before slicing.

To serve, garnish cake with chocolate curls or dust with cocoa if desired. Run a thin knife between the cake and sides of pan. Remove sides and run the cleaned knife along the outside to smooth. Cut using cheese wire or a hot knife, cleaning and drying between each cut.