Preheat your oven to 350ºF (175ºC). Grease an 8-inch springform pan with nonstick spray and line the bottom with parchment paper.

Melt 6 ounces of unsweetened chocolate and ½ cup of unsalted butter in a medium saucepan over medium-low heat. Once smooth, remove from heat and whisk in 1 cup of granulated sugar. Let it cool slightly before adding 3 large eggs and 1 tablespoon of vanilla extract.

Sift in 1 cup of flour, 3 tablespoons of cocoa powder, and ½ teaspoon of salt. Mix until just combined, then pour into the prepared pan and bake for 20-25 minutes until a toothpick comes out with moist crumbs. Allow the brownie base to cool completely.

In a small bowl, sprinkle ¾ teaspoon of gelatin over 1 tablespoon of water and let it soften. In a saucepan, heat 1 ½ cups of heavy cream with 2 tablespoons of sugar until dissolved. Remove from heat and stir in the softened gelatin until dissolved. Pour over 6 ounces of chopped semisweet chocolate, whisking until smooth. Cool at room temperature.

In a separate bowl, whip the remaining cream until it reaches medium-stiff peaks. Gently fold the whipped cream into the chocolate mixture until no streaks remain. Spread the mousse evenly atop the cooled brownie and refrigerate for at least 2 hours to set.

In a saucepan, heat ¾ cup of heavy cream and 1 tablespoon of unsalted butter until it simmers. Remove from heat and pour over 9 ounces of chopped semisweet chocolate. Stir until the chocolate melts and the mixture is smooth.

Allow the ganache to cool to room temperature before spreading it over the set mousse layer. Chill the cake for about 1 hour until the ganache is fully set. Carefully remove the sides of the springform pan.

Slice the cake and serve with fresh berries or a dollop of whipped cream.