Preheat the oven to 350ºF. Lightly spray an 8-inch springform pan with nonstick spray and line with parchment paper.

In a medium saucepan, combine chocolate and butter over medium-low heat, stirring until melted and smooth. Remove from heat and whisk in sugar. Let cool slightly.

Whisk in the eggs and vanilla until combined.

Sift in flour, cocoa powder, and salt, and fold until no dry streaks are visible. Spread the batter evenly in the prepared pan and bake for 20-25 minutes. Allow to cool completely, or refrigerate for 15-20 minutes.

In a small bowl, sprinkle gelatin over water and let stand for 5 minutes. In a small saucepan, heat ½ cup cream and 2 tablespoons sugar until simmering. Remove from heat, add softened gelatin, and stir until dissolved. Pour over the chocolate and whisk until smooth. Let cool to room temperature.

Whip remaining cream at medium speed until thick. Gradually mix in ⅓ of the whipped cream with the chocolate mixture, then fold in the remaining whipped cream until no streaks remain.

Spread mousse over the cooled brownie layer and refrigerate while preparing ganache.

Heat cream and butter in a medium saucepan over medium-high heat until just before boiling. Pour over chocolate in a bowl and stir until melted and smooth. Let cool until no longer warm.

Spread ganache over mousse layer and refrigerate until set, about 1 hour.

Remove sides of the springform pan and carefully peel off parchment paper. Transfer to a serving platter.