Make the Brownie:Preheat the oven to 350ºF (175ºC). Lightly spray an 8-inch springform pan with nonstick spray and line with parchment paper.
In a saucepan, melt the chocolate and butter over medium-low heat, stirring until smooth. Add sugar and whisk until combined. Let cool slightly.
Whisk in the eggs and vanilla extract. Sift in flour, cocoa powder, and salt, then fold gently until just combined. Spread the batter into the prepared pan.
Bake for 20-25 minutes, until a toothpick inserted comes out with a few moist crumbs. Let the brownie cool completely.
Make the Mousse: In a small bowl, sprinkle gelatin over water and let stand for 5 minutes. In a separate bowl, place the chopped chocolate.
Heat ½ cup cream and sugar in a small saucepan over medium heat until it simmers. Remove from heat, add the gelatin, and stir until dissolved. Pour the cream mixture over the chocolate, whisking until smooth. Let cool to room temperature.
Whip the remaining 1 cup of cream until medium to stiff peaks form. Fold the whipped cream into the chocolate mixture in thirds until no streaks remain. Spread the mousse over the cooled brownie layer and refrigerate.
Make the Ganache:Heat the cream and butter in a saucepan over medium-high heat. Remove just before boiling. Pour the hot cream over the chopped chocolate and stir until smooth. Let cool until it’s no longer warm.
Spread the ganache over the mousse layer and refrigerate for 1 hour to set.
Remove the cake from the springform pan, peel off the parchment, and serve with whipped cream or berries.