For the Brownie Base:

1. Preheat your oven to 350°F (175°C) and grease or line an 8×8-inch baking pan with parchment paper.
2. In a mixing bowl, whisk together the melted butter and granulated sugar until smooth. Add the eggs and vanilla extract, then whisk until well combined.
3. Sift in the cocoa powder, flour, salt, and baking powder. Stir until just combined. The batter should be thick.
4. Pour the brownie batter into the prepared pan and spread it evenly.
5. Bake in the preheated oven for 20-25 minutes or until a toothpick inserted into the center comes out with a few moist crumbs (not wet batter). Let it cool completely in the pan.

For the Chocolate Mousse:

1. Place the chopped semisweet chocolate in a heatproof bowl. In a small saucepan, heat the milk and powdered sugar over medium heat until warm (but not boiling).
2. Pour the warm milk over the chopped chocolate and stir until the chocolate is completely melted and smooth. Let it cool to room temperature.
3. In a separate mixing bowl, beat the heavy whipping cream until stiff peaks form.
4. Gently fold the cooled chocolate mixture into the whipped cream until fully combined and smooth. Be careful not to deflate the whipped cream.

For the Ganache Topping:

1. In a heatproof bowl, place the chopped semisweet chocolate.
2. In a small saucepan, heat the heavy cream until it begins to simmer, then pour it over the chocolate. Let it sit for 2 minutes before stirring until smooth and glossy.

Assembling the Brownies:

1. Once the brownie base has cooled, spread the chocolate mousse layer evenly over the top of the brownies.
2. Pour the ganache over the mousse layer, spreading it evenly to cover the entire surface.
3. Refrigerate the brownies for at least 2 hours to allow the mousse and ganache to set.