Preheat your oven to 350°F (175°C) and line an 8×8-inch (20×20 cm) baking pan with parchment paper for easy removal.

In a large mixing bowl, combine the melted butter, granulated sugar, eggs, and vanilla extract, whisking until the mixture is smooth and well blended.

Sift the cocoa powder, flour, salt, and baking powder into the wet ingredients. Gently fold everything together using a spatula or wooden spoon, mixing just until no dry streaks remain—be careful not to overmix.

Pour the batter into the prepared pan and spread it evenly.

Bake for 20 to 25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Allow the brownie layer to cool completely in the pan before adding the mousse.

In a small saucepan, heat the milk until steaming (not boiling), then pour it over the finely chopped semisweet chocolate in a heatproof bowl. Let it sit undisturbed for 2 minutes, then stir until smooth and fully melted. Set the mixture aside to cool to room temperature—it should be fluid but no longer warm.

In a separate bowl, whip the chilled heavy cream with the powdered sugar until soft peaks form.

Carefully fold the cooled chocolate mixture into the whipped cream in batches, being gentle to preserve the light texture of the mousse.

Spread the mousse evenly over the cooled brownie base, smoothing the top. Chill in the refrigerator for at least 2 hours, or until the mousse layer is firm.

Place the finely chopped semisweet chocolate in a heatproof bowl. In a small saucepan, heat the heavy cream until it starts to simmer, then pour it over the chocolate. Let it stand for 2 to 3 minutes, then stir until the mixture is silky and glossy.

Pour the ganache over the set mousse layer and use an offset spatula to spread it evenly to the edges.

Return the pan to the refrigerator and chill for at least 1 more hour, or until the ganache is fully set and firm to the touch.