Preheat your oven to 350°F (175°C) and grease an 8-inch springform pan with butter. Melt together the unsweetened chocolate and butter in a heatproof bowl over simmering water, stirring until smooth. Remove from heat, mix in granulated sugar, then whisk in the eggs and vanilla extract. Fold in all-purpose flour, cocoa powder, and salt just until combined, then pour the batter into the prepared pan and bake for 20-25 minutes, or until a toothpick comes out clean.

While the brownie base bakes and cools, bloom the gelatin by sprinkling it over 2 tablespoons of water, letting it sit for about 5 minutes. In a saucepan, heat 1 cup of heavy cream until it's warm but not boiling; remove from heat and stir in the bloomed gelatin until dissolved. In a separate bowl, melt the semisweet chocolate, then pour in the gelatin mixture, mixing until combined. Whip the remaining heavy cream to medium-stiff peaks, then gently fold it into the chocolate mixture to create a light mousse.

Once the brownie base has fully cooled, gently pour the mousse over it, using a spatula to spread it evenly. Tap the pan lightly on the countertop to remove any air pockets. Cover the cake with plastic wrap and refrigerate for at least 4 hours, or until set.

When the mousse has set, make the ganache by heating 1 cup of heavy cream along with the unsalted butter in a small saucepan over medium heat. Stir until the mixture is hot and the butter has melted but not boiling. Remove from heat, pour it over the chopped semisweet chocolate in a bowl, and allow it to sit for 2-3 minutes to melt the chocolate. Whisk until smooth and glossy.

Pour the ganache over the chilled mousse layer, using a spatula to spread it evenly across the surface. Refrigerate the entire cake once more for about 1 hour, allowing the ganache to set firmly.

After the ganache has set, carefully remove the sides of the springform pan. Transfer the cake to a serving platter. For the best texture, let the cake sit at room temperature for 30 minutes before slicing. Serve generous slices and watch as everyone delights in the layers of chocolatey goodness.