Part 1: Bake the Fudgy Brownie Layer

1. 1. Preheat and Prep Pan: Preheat oven to 325°F (160°C). Line a 13×9-inch pan with parchment paper, leaving an overhang on the long sides. Grease the parchment and pan with the 2 tablespoons of softened butter.
   2. Mix Batter: In a large bowl, beat the eggs on medium speed until fluffy. Add the sugar and beat until just combined. Add the melted butter, sifted cocoa powder, vanilla, sifted flour, and salt. Mix on low speed only until the batter is just combined.
   3. Bake and Cool: Pour the batter into the prepared pan and spread evenly. Bake for 30-35 minutes, until a toothpick inserted in the center comes out with just a few moist crumbs. Do not overbake. Let the brownie cool completely in the pan on a wire rack.

Part 2: Make the Silky Chocolate Mousse

1. Melt Chocolate: Place the chopped milk chocolate in a large, heatproof bowl over a pot of simmering water (double boiler), ensuring the bowl doesn’t touch the water. Stir until melted and smooth. Remove from heat.
2. Bloom & Dissolve Gelatin: Sprinkle the gelatin packet over the 2 tablespoons of cold water in a small, microwave-safe bowl. Let it “bloom” for 10 minutes. In a small saucepan, bring ½ cup of the heavy cream to a gentle boil. Remove from heat. Add the bloomed gelatin and stir until fully dissolved.
3. Combine Chocolate & Cream: Pour half of the hot cream/gelatin mixture into the melted chocolate and whisk vigorously. Add the rest of the hot cream and whisk until the mixture is perfectly smooth and glossy. Set this “ganache” aside to cool completely to room temperature.
4. Whip Cream & Fold Mousse: In a large, chilled bowl, whip the remaining 2 cups of cold heavy cream on medium-high speed until it forms soft peaks. Add about one-third of the whipped cream to the cooled chocolate ganache and fold it in gently to lighten it. Add the remaining whipped cream and gently fold until no white streaks remain.

Part 3: Assemble and Chill

1. Top the Brownies: Pour the finished chocolate mousse evenly over the completely cooled brownie base in the pan. Spread it into a smooth, level top layer.
2. Chill to Set: Cover the pan with plastic wrap (try not to let it touch the mousse) and refrigerate for at least 6 hours, or preferably overnight, until the mousse is fully set.
3. Slice and Serve: Use the parchment paper overhangs to lift the entire brownie slab out of the pan. For clean cuts, dip a large, sharp knife in hot water, wipe it dry, and make one slice. Repeat dipping and wiping before every single cut. Serve chilled.