1. Make the Brownie Base

The foundation of this cake is a rich, fudgy brownie layer. Here’s how to prepare it:

1. Prepare the pan: Preheat your oven to 350°F (175°C). Lightly spray an 8-inch springform pan with nonstick spray. Line the bottom and sides with parchment paper, and spray again for easy release.
2. Melt the chocolate and butter: In a medium saucepan, melt the unsweetened chocolate and butter over medium-low heat, stirring constantly until smooth. Remove from heat and whisk in the sugar. Allow the mixture to cool slightly, ensuring it’s not too hot to scramble the eggs.
3. Incorporate the wet ingredients: Whisk in the eggs one at a time, followed by the vanilla extract.
4. Combine dry ingredients: Sift together the flour, cocoa powder, and salt. Gently fold the dry mixture into the wet ingredients using a rubber spatula. Avoid overmixing.
5. Bake: Spread the batter evenly in the prepared pan. Bake for 20–25 minutes, or until a toothpick inserted in the center comes out with moist crumbs. Cool the brownie completely in the pan on a wire rack or refrigerate it for faster cooling.

2. Prepare the Mousse Layer

The star of the cake, this mousse is light, creamy, and full of chocolate flavor.

1. Bloom the gelatin: In a small bowl, sprinkle gelatin over water and let it sit for 5 minutes to bloom.
2. Heat the cream and sugar: In a small saucepan, heat ½ cup of heavy cream with the sugar over medium heat until it just begins to simmer. Stir to dissolve the sugar.
3. Melt the chocolate: Pour the hot cream mixture over the chopped semisweet chocolate and add the bloomed gelatin. Stir until the chocolate is fully melted and the mixture is smooth. Cool to room temperature, stirring occasionally to prevent a skin from forming.
4. Whip the cream: In a stand mixer fitted with a whisk attachment, whip the remaining 1 cup of heavy cream until medium-stiff peaks form.
5. Incorporate the cream: Whisk ⅓ of the whipped cream into the cooled chocolate mixture to lighten it. Gently fold in the remaining whipped cream with a spatula until no streaks remain.
6. Assemble the mousse layer: Spread the mousse evenly over the cooled brownie layer. Refrigerate the cake while you prepare the ganache.

3. Make the Ganache Topping

The finishing touch is a glossy ganache that seals in all the layers.

1. Heat the cream and butter: In a medium saucepan, heat the heavy cream and butter over medium heat until just before boiling.
2. Melt the chocolate: Pour the hot cream mixture over the chopped chocolate in a heatproof bowl. Stir until smooth and silky. Allow the ganache to cool to room temperature before using.
3. Top the cake: Spread the ganache evenly over the mousse layer. Refrigerate for at least 1 hour, or until fully set.