Preheat oven to 350°F (175°C) and prepare an 8x8-inch pan by greasing or lining with parchment paper.

Whisk melted butter and sugar together in a mixing bowl until creamy. Add eggs and vanilla extract. In a separate bowl, mix cocoa powder, flour, baking powder, and salt. Combine wet and dry mixtures until just blended.

Pour brownie batter into prepared pan and spread evenly. Bake for 20-25 minutes or until a toothpick comes out with a few moist crumbs. Allow to cool completely on a wire rack.

Melt semisweet chocolate and milk in a saucepan, stirring until smooth. Whip the chilled cream with powdered sugar to stiff peaks and fold in the melted chocolate mixture.

Heat heavy cream for ganache in a small saucepan until simmering. Pour over chopped chocolate and let sit for a minute before stirring until smooth.

Spread the mousse evenly over the cooled brownies and drizzle the ganache on top. Refrigerate for at least 2 hours to set.