Make the Brownie Base:

1. Preheat the Oven:
   Preheat your oven to 350°F (175°C). Grease or line an 8×8-inch baking pan with parchment paper.

2. Prepare the Brownie Batter:
   In a medium-sized bowl, whisk together the melted butter and granulated sugar until smooth.
   Add the eggs and vanilla extract, mixing until fully combined.
   In a separate bowl, sift together the cocoa powder, flour, salt, and baking powder. Gradually add the dry ingredients into the wet ingredients, stirring until just combined.

3. Bake the Brownies:
   Pour the brownie batter into the prepared pan and spread it evenly.
   Bake for 20-25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Allow the brownies to cool completely in the pan on a wire rack.

Make the Chocolate Mousse:

1. Melt the Chocolate:
   In a microwave-safe bowl, heat the semisweet chocolate and milk in 30-second intervals, stirring in between, until the chocolate is completely melted and smooth.

2. Whip the Cream:
   In a separate bowl, whip the chilled heavy cream with powdered sugar until stiff peaks form.

3. Combine the Mousse:
   Gently fold the melted chocolate into the whipped cream until fully combined, being careful not to deflate the whipped cream. The mousse should be smooth and fluffy.

Make the Ganache Topping:

1. Prepare the Ganache:
   In a small saucepan, heat the heavy cream over medium heat until it just begins to simmer.
   Pour the hot cream over the finely chopped semisweet chocolate in a bowl and let it sit for 2-3 minutes. Stir until smooth and glossy.

Assemble the Brownies:

1. Layer the Mousse:
   Once the brownies have cooled completely, spread the chocolate mousse evenly over the brownie base.

2. Top with Ganache:
   Drizzle the ganache over the mousse layer, spreading it evenly with a spatula or back of a spoon.

3. Chill:
   Place the brownies in the refrigerator to chill for at least 2 hours or until the mousse and ganache are firm and set.