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

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

Sift in the cocoa powder, flour, salt, and baking powder. Using a spatula, gently fold the dry ingredients into the wet mixture until just combined—avoid overmixing to keep the brownies tender.

Pour the batter into the lined baking pan and use a spatula to smooth out the surface evenly.

Bake for 20–25 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 moving on.

In a small saucepan, heat the milk until it is steaming but not boiling. Pour it over the finely chopped semisweet chocolate placed in a heatproof bowl. Let the mixture sit for about 2 minutes, then stir until the chocolate is melted and silky smooth. Allow it to cool to room temperature, remaining fluid but not warm.

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

Carefully fold the cooled chocolate mixture into the whipped cream, mixing gently to preserve the airy texture. Continue folding until the mousse is smooth and fully combined.

Spread the mousse evenly over the cooled brownie base. Place the pan in the refrigerator and chill for at least 2 hours, or until the mousse is firm and set.

Put the chopped semisweet chocolate into a heatproof bowl. In a small saucepan, heat the heavy cream just until it begins to simmer. Pour the hot cream over the chocolate and let it sit for 2–3 minutes. Stir until the ganache becomes smooth, glossy, and fully blended.

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

Refrigerate the assembled dessert for another hour, or until the ganache is fully set and firm to the touch.