Step 1: Make and Bake the Brownie Base

• Preheat your oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper, leaving an overhang on two sides for easy lifting later. Grease the parchment and any exposed pan sides.
• In a large mixing bowl, whisk together the melted butter and granulated sugar until combined.
• Whisk in the eggs one at a time, then stir in the vanilla extract until smooth.
• Sift the cocoa powder, flour, salt, and baking powder directly over the wet ingredients.
• Gently fold the dry ingredients into the wet ingredients using a rubber spatula until just combined. Be careful not to overmix.
• Pour the batter into the prepared pan and spread it into an even layer with the spatula.
• Bake for 20-25 minutes, or until a toothpick inserted near the center comes out with a few moist crumbs attached (you want it fudgy, not overbaked).
• Let the brownie cool completely in the pan on a wire rack.

Step 2: Prepare the Chocolate Mousse

• Melt Chocolate Base: Place the 8 oz of finely chopped semisweet chocolate in a medium heatproof bowl. Heat the 3 tablespoons of milk (or reserved heavy cream) in a small saucepan or microwave until just steaming (not boiling). Pour the hot milk/cream over the chopped chocolate. Let it sit undisturbed for 2 minutes. Gently stir until the chocolate is fully melted and the mixture is smooth. Let this chocolate mixture cool until it’s no longer warm to the touch but still fluid (approximately room temperature).
• Whip Cream: In a separate large, chilled bowl, whip the remaining chilled heavy whipping cream (approx. 1 1/4 cups + 1 tbsp) with the sifted powdered sugar using an electric mixer until soft peaks form (the peak flops over when beaters are lifted). Be careful not to overwhip into grainy territory.
• Combine Mousse: Gently fold about one-third of the whipped cream into the cooled chocolate mixture using a rubber spatula to lighten it. Then, carefully scrape the lightened chocolate mixture onto the remaining whipped cream. Gently fold everything together until just combined and no streaks remain. Be very careful not to deflate the air incorporated into the whipped cream.

Step 3: Assemble Brownie and Mousse Layers

• Ensure the brownie base is completely cool in the pan.
• Pour the prepared chocolate mousse evenly over the cooled brownie base.
• Use an offset spatula or the back of a spoon to spread the mousse into a smooth, even layer.
• Cover the pan loosely with plastic wrap and refrigerate for at least 2 hours, or until the mousse layer is well-chilled and set.

Step 4: Make and Add Ganache Topping

• Once the mousse layer is set, make the ganache. Place the 4 oz of finely chopped semisweet chocolate in a small heatproof bowl.
• Heat the 1/2 cup of heavy cream in a small saucepan over medium heat (or in the microwave) until it just begins to simmer around the edges.
• Pour the hot cream directly over the chopped chocolate. Let it sit undisturbed for 2-3 minutes.
• Gently whisk the mixture, starting from the center and working outwards, until the ganache is completely smooth and glossy.
• Let the ganache cool slightly for 5-10 minutes until it’s still pourable but not hot (it should be slightly thickened).
• Pour the slightly cooled ganache over the set mousse layer in the pan.
• Gently tilt the pan or use an offset spatula to spread the ganache into an even layer covering the entire mousse surface.

Step 5: Final Chill and Serving

• Refrigerate the pan again for at least another hour, or until the ganache layer is firm to the touch.
• Once fully chilled and set, use the parchment paper overhangs to carefully lift the entire block out of the pan onto a cutting board.
• Use a large, sharp knife, dipped in hot water and wiped clean between cuts, to slice into squares or bars.
• Serve chilled. Store leftovers covered in the refrigerator.