Preheat oven to 350°F (175°C) and line an 8x8-inch baking pan with parchment paper, leaving an overhang on two sides.

In a medium bowl, whisk together melted butter and granulated sugar until combined. Whisk in eggs and vanilla until smooth.

Sift cocoa powder, flour, and salt into the wet ingredients; fold gently with a spatula until no dry streaks remain.

Spread batter evenly into the pan and bake for 22-25 minutes until the center is set. Cool completely in the pan.

Prepare the mousse stabilizer: Sprinkle gelatin over 2 tablespoons cold water and let bloom for 5 minutes, then microwave for 10 seconds until liquid.

Melt 8 ounces of chopped chocolate in a heat-proof bowl. Stir in the liquid gelatin until smooth and set aside to cool slightly.

In a separate chilled bowl, whip 1.5 cups of heavy cream and powdered sugar to stiff peaks.

Gently fold one-third of the whipped cream into the melted chocolate to lighten the mixture, then fold in the remaining cream until uniform.

Spread the mousse over the cooled brownie layer and refrigerate for at least 4 hours to set.

Make the ganache: Heat the remaining 1/2 cup heavy cream until simmering, then pour over the remaining 4 ounces of chopped chocolate.

Let the ganache sit for 3 minutes, whisk until glossy, pour over the chilled mousse layer, and refrigerate for 30 minutes before slicing.