1-Preheat the oven to 350°F (175°C) and: Preheat the oven to 350°F (175°C) and line an 8×8-inch (20×20 cm) square baking pan with parchment paper, leaving a slight overhang on the sides for easy removal.
2-In a large bowl, whisk together the: In a large bowl, whisk together the melted butter, granulated sugar, eggs, and vanilla extract until smooth and well combined.
3-Sift in the cocoa powder, flour, salt,: 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.
4-Pour the batter into the prepared pan: 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.
5-Heat the milk in a small saucepan: 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.
6-In a separate bowl, beat the chilled: In a separate bowl, beat the chilled heavy cream and powdered sugar until soft peaks form.
7-Stir a spoonful of whipped cream into: 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.
8-Spread the chocolate mousse evenly over the: 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.
9-Place the chopped chocolate in a heatproof: 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.
10-Pour the ganache over the chilled mousse: 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.
11-Once fully set, lift the dessert out: 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.