Preheat your oven to 350°F (175°C) and line an 8×8-inch (20×20 cm) baking pan with parchment paper. In a mixing bowl, whisk together the melted butter, granulated sugar, eggs, and vanilla until smooth. Sift in the cocoa powder, flour, salt, and baking powder. Gently fold the dry ingredients into the wet ingredients until just combined. Avoid overmixing. Pour the batter into the prepared pan and smooth it out with a spatula. Bake for 20-25 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Let the brownie cool completely in the pan.

Heat the milk in a small saucepan until just steaming (but not boiling). Pour it over the chopped semisweet chocolate in a heatproof bowl. Let it sit for 2 minutes, then stir until the chocolate is fully melted and smooth. Let it cool until it’s no longer warm but still fluid (about room temperature). In a separate bowl, whip the heavy cream with the powdered sugar until soft peaks form. Gently fold the cooled chocolate mixture into the whipped cream until fully incorporated. Be careful not to deflate the mousse. Spread the mousse evenly over the cooled brownie base. Refrigerate 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, then pour it over the chocolate. Let it sit for 2-3 minutes, then stir until smooth and glossy. Pour the ganache over the mousse layer and spread it evenly with an offset spatula. Refrigerate for another hour, or until the ganache is firm.