Preheat your oven to 350°F (175°C) and line an 8x8 inch pan with parchment paper. In a mixing bowl, melt unsalted butter and whisk in granulated sugar until combined. Add eggs and vanilla extract, stirring until smooth. Carefully fold in cocoa powder, flour, salt, and baking powder until mixed. Pour batter into the prepared pan and bake for 20 minutes or until a toothpick comes out with a few moist crumbs. Allow cooling on a wire rack.

In a saucepan, heat milk until steaming, then pour it over finely chopped semisweet chocolate. Stir gently until the chocolate melts and becomes smooth; let it cool to room temperature. Whip heavy cream with powdered sugar until soft peaks form, then fold in the cooled chocolate mixture.

Spread the mousse evenly over the cooled brownie base using a spatula. Cover the pan with plastic wrap and refrigerate for at least 2 hours.

Heat heavy cream in a saucepan until it just begins to bubble, then pour it over finely chopped semisweet chocolate. Stir until a silky ganache forms.

Drizzle the ganache over the chilled mousse layer and return to the refrigerator for at least 1 hour.

Once set, slice the brownies with a warm knife and serve chilled.