Prepare the Brownie Base:

• Preheat the oven to 350°F (175°C). Grease an 8×8-inch baking pan or line it with parchment paper.
• In a large bowl, mix the melted butter and granulated sugar until smooth.
• Add the eggs, one at a time, followed by the vanilla extract, and mix until well combined.
• In a separate bowl, sift together the cocoa powder, flour, salt, and baking powder. Gradually add the dry ingredients to the wet ingredients and mix until just combined.
• Pour the brownie batter into the prepared baking pan, spreading it evenly.
• Bake for 20-25 minutes, or until a toothpick inserted into the center comes out with a few moist crumbs. Let the brownies cool completely in the pan.

Make the Chocolate Mousse:

• Place the chopped semisweet chocolate into a heatproof bowl.
• In a small saucepan, heat the milk over medium heat until just simmering (not boiling). Pour the hot milk over the chopped chocolate and stir until the chocolate is completely melted and smooth. Let it cool to room temperature.
• In a separate chilled bowl, whip the heavy cream and powdered sugar using an electric mixer until soft peaks form.
• Gently fold the melted chocolate mixture into the whipped cream until smooth and fully incorporated. Be careful not to deflate the whipped cream.

Assemble the Brownies:

• Once the brownies have cooled, spread the chocolate mousse evenly over the brownie base. Use a spatula to smooth the top.
• Refrigerate the brownies for at least 2 hours to set the mousse layer.

Prepare the Ganache Topping:

• In a small saucepan, heat the heavy cream over medium heat until it begins to simmer.
• Pour the hot cream over the chopped semisweet chocolate and let it sit for 2-3 minutes. Stir until the chocolate is completely melted and the ganache is smooth.
• Pour the ganache over the chilled mousse layer, spreading it evenly over the top.

Chill and Serve:

• Refrigerate the layered brownies for at least 2 more hours, or until the ganache is set.
• Once set, cut into squares and serve. Enjoy!