Prepare the Brownie Base:

• Preheat the oven to 350°F (175°C) and grease or line an 8×8-inch baking pan with parchment paper.
• In a medium-sized mixing bowl, whisk together the melted butter and sugar until combined.
• Add the eggs and vanilla extract, and whisk until smooth.
• In another bowl, combine the cocoa powder, flour, salt, and baking powder. Gradually add the dry ingredients into the wet mixture, stirring until just combined.
• Pour the brownie batter into the prepared pan and spread it out evenly.
• Bake for 20-25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs (not wet batter). Let the brownies cool completely in the pan.

Make the Chocolate Mousse:

• In a heatproof bowl, melt the semisweet chocolate with the milk. You can do this over a double boiler or in the microwave in 30-second intervals, stirring until smooth. Let the chocolate cool slightly.
• In a separate bowl, beat the chilled heavy whipping cream and powdered sugar using an electric mixer until stiff peaks form.
• Gently fold the cooled melted chocolate into the whipped cream until fully combined. Be careful not to deflate the whipped cream.

Prepare the Ganache Topping:

• In a heatproof bowl, combine the semisweet chocolate and heavy cream. Microwave in 30-second intervals, stirring in between, until the chocolate is melted and smooth. Let the ganache cool slightly before using.

Assemble the Brownies:

• Once the brownie base has cooled completely, spread a thick layer of chocolate mousse evenly over the top.
• Drizzle the ganache over the mousse layer and use a spatula to smooth it out into an even layer. For an extra touch, you can swirl the ganache into the mousse for a marbled effect.
• Refrigerate the brownies for at least 2 hours, or until the mousse and ganache have set.

Serve:

• Once set, cut the brownies into squares and serve chilled. Optionally, garnish with shaved chocolate or whipped cream.