Decadent Triple-Chocolate Mousse Torte

1. Prepare the Bottom Layer

Begin by adjusting your oven rack to the middle position and preheating your oven to 325°F. Grease a 9 1/2-inch springform pan with butter. In a heatproof bowl set over a saucepan of simmering water, combine the butter, 7 ounces of the bittersweet chocolate, and the espresso powder. Stir until the mixture is smooth, then set it aside to cool slightly. Once cooled, whisk in the vanilla and egg yolks until well combined. In a stand mixer fitted with the whisk attachment, beat the egg whites and a pinch of salt on medium speed until frothy. Gradually add half of the brown sugar and continue to beat until soft peaks form. Gently fold in the remaining brown sugar and the chocolate mixture until fully incorporated. Pour the batter into the prepared pan and bake for 13 to 18 minutes, or until the cake springs back when pressed gently in the center. Allow the cake to cool in the pan on a wire rack for about an hour.

2. Create the Middle Layer

In a small bowl, combine the cocoa powder and hot water, then set aside. Using the same method as before, melt the remaining 7 ounces of bittersweet chocolate in a heatproof bowl set over a saucepan of simmering water, stirring until smooth. Set the chocolate aside to cool slightly, which should take about 2 to 5 minutes. In a stand mixer fitted with the whisk attachment, whip 1.5 cups of the cold cream, granulated sugar, and a pinch of salt on medium speed until the mixture thickens. Increase the speed to high and whip until soft peaks form, which should take 30 to 60 seconds. Whisk the cocoa mixture into the melted chocolate until smooth, then gently fold the chocolate mixture into the whipped cream with a rubber spatula. Spoon the mousse over the cooled bottom layer, then tap the pan gently on the counter to remove any air bubbles. Smooth the top with a spatula and refrigerate for about 15 minutes.

3. Finish with the Top Layer

In a small bowl, sprinkle the gelatin over the water and let it stand for about 5 minutes to soften. Heat 0.5 cup of the cream in a small saucepan over medium heat until just simmering. Add the gelatin mixture and stir until it completely dissolves. Pour the hot cream mixture over the white chocolate in a medium bowl, and whisk until the chocolate is melted and the mixture is smooth. Allow the mixture to cool to room temperature, stirring occasionally, which should take about 5 to 8 minutes. In a stand mixer fitted with the whisk attachment, whip the remaining 1 cup of cold cream on medium speed until it thickens. Increase the speed to high and whip until soft peaks form, which should take 30 to 60 seconds. Gently fold the whipped cream into the white chocolate mixture with a rubber spatula until no streaks remain. Spoon the white chocolate mousse over the middle layer, smooth the top with a spatula, and chill the torte for at least 2.5 hours to set.

4. To Serve

If desired, garnish the torte with shaved chocolate or a dusting of cocoa powder. Run a thin knife around the edge of the torte to loosen it, then remove the sides of the springform pan. Smooth the sides of the torte with a spatula if needed. Cut the torte into slices using a sharp knife, wiping the blade clean between each cut, and serve.