Preheat and Prepare: Preheat the oven to 350°F (175°C). Line a 9x13-inch baking pan with parchment paper.
Make the Brownie Batter: In a large bowl, whisk together melted butter, granulated sugar, and vanilla extract. Add eggs one at a time, whisking well after each addition.
Incorporate Dry Ingredients: Sift in all-purpose flour, cocoa powder, and salt. Gently fold the mixture until fully combined.
Bake the Brownie Layer: Pour the batter into the prepared baking pan and spread evenly. Bake for 30-35 minutes or until a toothpick inserted in the center comes out with a few moist crumbs. Let the brownies cool completely before adding the mousse layer.
Prepare the Gelatin: In a small bowl, mix cold water with unflavored gelatin and let it sit for 5 minutes to bloom.
Melt the Chocolate: In a saucepan, heat ½ cup of heavy cream over low heat. Add the chopped semi-sweet chocolate and stir until fully melted and smooth. Remove from heat.
Incorporate the Gelatin: Microwave the bloomed gelatin for 5-10 seconds until liquefied, then stir it into the melted chocolate mixture. Let cool slightly.
Whip the Cream: In a large bowl, beat the remaining 2 cups of heavy cream with powdered sugar and vanilla extract until soft peaks form.
Combine and Layer: Gently fold the whipped cream into the melted chocolate mixture until smooth. Spread the chocolate mousse evenly over the cooled brownie layer.
Chill and Set: Refrigerate for at least 4 hours or until the mousse is fully set.
Garnish and Serve: Lightly dust the top with cocoa powder before slicing and serving.
