Step 1: Prepare the Bottom Layer

• Preheat your oven to 325°F (165°C). Grease a 9-inch springform pan and line the bottom with parchment paper.
• In a heatproof bowl, combine the butter, chopped bittersweet chocolate, and espresso powder. Melt the mixture over a double boiler or in the microwave in 20-second intervals, stirring until smooth.
• Remove from heat and whisk in the vanilla extract and egg yolks until fully combined.
• In a separate bowl, beat the egg whites and salt until frothy. Gradually add the brown sugar, whipping until soft peaks form.
• Gently fold the whipped egg whites into the chocolate mixture in three additions, being careful not to deflate the batter.
• Pour the batter into the prepared pan and bake for 15–18 minutes, or until set. Let the bottom layer cool completely.

Step 2: Create the Middle Layer

• In a small bowl, whisk the cocoa powder and hot water until smooth.
• Melt the bittersweet chocolate over a double boiler or in the microwave, stirring until smooth. Allow it to cool slightly.
• In a chilled bowl, whip the cold heavy cream, granulated sugar, and salt until soft peaks form.
• Fold the cocoa mixture into the melted chocolate, then gently fold in the whipped cream in three additions.
• Spread the mousse evenly over the cooled bottom layer and refrigerate for at least 30 minutes to set.

Step 3: Make the Top Layer

• Sprinkle the powdered gelatin over 1 tablespoon of water in a small bowl and let it sit for 5 minutes to bloom.
• Melt the white chocolate chips over a double boiler or in the microwave, stirring until smooth.
• Heat the bloomed gelatin in the microwave for 10–15 seconds until fully dissolved, then stir it into the melted white chocolate.
• In a chilled bowl, whip the cold heavy cream until soft peaks form. Gently fold the whipped cream into the white chocolate mixture in three additions.
• Spread the white chocolate mousse evenly over the middle layer, smoothing the top with a spatula. Refrigerate the cake for at least 2 hours or preferably overnight to set completely.

Step 4: Garnish and Serve

• Dust the top of the cake with cocoa powder, sprinkle with shaved chocolate, or drizzle with melted chocolate for an elegant finish.
• Carefully release the cake from the springform pan. Use a warm knife to slice clean pieces and serve chilled.