Prepare the Brownie Base:
Preheat your oven to 350°F (175°C). Grease and line a 9×9-inch baking pan with parchment paper.
In a bowl, whisk together melted butter and granulated sugar until smooth. Add eggs, one at a time, followed by the vanilla extract.
In a separate bowl, mix together cocoa powder, flour, salt, and baking powder. Gradually fold the dry ingredients into the wet ingredients.
Pour the batter into the prepared pan and spread evenly. Bake for 20-25 minutes or until a toothpick inserted comes out mostly clean (a few moist crumbs are fine). Let it cool completely in the pan.

Make the Chocolate Mousse:
In a heatproof bowl, combine semisweet chocolate and milk. Melt over a double boiler or in the microwave, stirring occasionally until smooth. Let the melted chocolate cool to room temperature.
In a separate chilled bowl, beat the heavy whipping cream and powdered sugar until stiff peaks form. Gently fold the whipped cream into the cooled chocolate until smooth and well combined.
Spread the mousse evenly over the cooled brownie base and refrigerate for at least 2 hours, or until set.

Make the Ganache Topping:
In a heatproof bowl, combine chopped semisweet chocolate and heavy cream. Melt together over a double boiler or in the microwave, stirring frequently, until smooth. Let it cool slightly before pouring over the mousse layer.
Pour the ganache evenly over the mousse layer, smoothing it out with a spatula. Return the dessert to the refrigerator for another hour to allow the ganache to set.