1. Make the Brownie Base

• Preheat the Oven: Set your oven to 350ºF (175ºC). Prepare an 8-inch springform pan by spraying the bottom and sides with nonstick spray. Line the bottom and sides of the pan with parchment paper, and lightly spray the paper with more nonstick spray.

• Melt the Chocolate and Butter: In a medium saucepan, combine the unsweetened chocolate and butter. Warm over medium-low heat, stirring constantly until the chocolate melts completely and the mixture becomes smooth. Remove from heat and whisk in the granulated sugar.

• Incorporate the Eggs: Once the chocolate mixture has cooled slightly (it should still be warm but not hot), whisk in the eggs and vanilla extract until fully combined.

• Add the Dry Ingredients: Sift the flour, cocoa powder, and salt into the chocolate mixture. Use a rubber spatula to fold the ingredients together until just combined—be careful not to overmix.

• Bake the Brownie: Spread the batter evenly into the prepared pan. Bake for 20-25 minutes, or until a toothpick inserted into the center comes out with only a few moist crumbs. Allow the brownie to cool completely in the pan on a wire rack. You can speed up the cooling process by placing it in the refrigerator for 15-20 minutes.

2. Make the Mousse Filling

• Prepare the Gelatin: In a small bowl, sprinkle the gelatin over the water and let it stand for 5 minutes to bloom.

• Heat the Cream and Melt the Chocolate: In a small saucepan, combine ½ cup of heavy cream and 2 tablespoons of sugar. Bring to a simmer over medium-high heat, stirring to dissolve the sugar. Once simmering, remove from heat and add the bloomed gelatin. Stir until the gelatin is completely dissolved. Pour this mixture over the semisweet chocolate in a medium bowl and whisk until smooth.

• Cool the Chocolate Mixture: Allow the chocolate mixture to cool to room temperature, stirring occasionally. It will thicken slightly as it cools.

• Whip the Cream: In the bowl of a stand mixer (or using a handheld electric mixer), whip the remaining 1 cup of heavy cream on medium speed until it begins to thicken. Increase the speed to high and whip until medium to stiff peaks form (about 30-60 seconds).

• Fold the Mousse: Whisk ⅓ of the whipped cream into the cooled chocolate mixture to lighten it. Then, gently fold the remaining whipped cream into the chocolate mixture using a rubber spatula until no streaks remain.

• Layer the Mousse: Spread the mousse evenly over the cooled brownie layer. Refrigerate while you prepare the ganache.

3. Make the Ganache Topping

• Heat the Cream and Butter: In a medium saucepan, heat the heavy cream and butter over medium-high heat. Remove from heat just before the mixture comes to a boil.

• Melt the Chocolate: Place the semisweet chocolate in a heatproof bowl and pour the hot cream mixture over it. Stir until the chocolate is completely melted and the ganache is smooth.

• Cool the Ganache: Set the ganache aside to cool until it is no longer warm to the touch.

• Top the Cake: Once the ganache has cooled slightly, spread it evenly over the mousse layer. Refrigerate the cake until the ganache is set, about 1 hour.

4. Finishing Touches

• Remove the Cake from the Pan: Carefully remove the sides of the springform pan. Gently peel off the parchment paper from the edges and transfer the cake to a serving platter.

• Serve: The cake is best served chilled. Let it sit at room temperature for about 30 minutes before slicing to allow the mousse and ganache to soften slightly, making the texture even more luxurious. Garnish with whipped cream or fresh berries if desired.