Preheat the oven to 350°F (175°C) and line an 8x8-inch (20x20 cm) square baking pan with parchment paper, leaving a slight overhang on the sides for easy removal.

In a large bowl, whisk together the melted butter, granulated sugar, eggs, and vanilla extract until smooth and well combined.

Sift in the cocoa powder, flour, salt, and baking powder. Gently fold the dry mixture into the wet ingredients just until no streaks remain. Do not overmix.

Pour the batter into the prepared pan and smooth the top with a spatula. Bake for 20 to 25 minutes until a toothpick inserted into the center comes out with a few moist crumbs. Let the brownie base cool completely in the pan.

Heat the milk in a small saucepan over low heat until just steaming. Pour it over the chopped chocolate in a heatproof bowl. Let it sit undisturbed for 2 minutes, then stir until fully melted and smooth. Let the mixture cool to room temperature.

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

Stir a spoonful of whipped cream into the cooled chocolate mixture to loosen it. Gently fold the remaining whipped cream into the chocolate until fully blended and airy. Avoid deflating the mousse.

Spread the chocolate mousse evenly over the cooled brownie base using a spatula. Refrigerate the pan for at least 2 hours, or until the mousse is set.

Place the chopped chocolate in a heatproof bowl. Heat the heavy cream in a small saucepan until it begins to simmer. Pour the hot cream over the chocolate and let sit for 2 to 3 minutes. Stir until smooth and glossy.

Pour the ganache over the chilled mousse layer and smooth the surface with an offset spatula. Return the pan to the refrigerator for 1 hour, or until the ganache is firm.

Once fully set, lift the dessert out of the pan using the parchment overhang. Slice with a clean, sharp knife, wiping between cuts for neat edges.