Preheat your oven to 350°F (175°C). Grease your 9x13-inch baking pan with the softened unsalted butter, ensuring every corner is coated.
In a large mixing bowl, combine the melted unsalted butter and white granulated sugar. Whisk until well combined. Add the eggs one at a time, mixing thoroughly after each addition. Stir in the vanilla extract.
Sift the unsweetened cocoa powder, flour, and kosher salt into the wet mixture. Gently fold the dry ingredients into the batter until just combined.
Pour the brownie batter into the prepared pan, spreading it evenly. Bake for 25-30 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Allow the brownies to cool completely in the pan.
While the brownies are cooling, bloom the gelatin in a small bowl with 2 tablespoons of water for about 5 minutes.
In a double boiler or microwave-safe bowl, melt the finely chopped milk chocolate until smooth. Allow it to cool slightly, then stir in the bloomed gelatin until fully dissolved.
In a separate bowl, whip the chilled whipping cream until stiff peaks form. Carefully fold the melted chocolate mixture into the whipped cream until no streaks remain.
Once the brownies are completely cool, pour the chocolate mousse over the top, spreading it evenly. Refrigerate for at least 2 hours to allow the mousse to set.
When you're ready to indulge, slice the brownies into squares and serve.
