Step 1: Make the Brownie Base
Preheat your oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper and set aside.

In a mixing bowl, whisk together the melted butter, granulated sugar, eggs, and vanilla extract until smooth and well combined.

Sift in the cocoa powder, flour, salt, and baking powder. Gently fold the dry ingredients into the wet ingredients just until combined. Avoid overmixing to keep the brownies fudgy.

Pour the batter into the prepared pan and smooth the top with a spatula. Bake for 20–25 minutes, or until a toothpick comes out with a few moist crumbs.

Let the brownie layer cool completely in the pan before adding the mousse.

Step 2: Prepare the Chocolate Mousse
In a small saucepan, heat the milk until just steaming. Pour it over the chopped chocolate in a heatproof bowl. Let sit for 2 minutes, then stir until completely melted and smooth.

Let the melted chocolate cool to room temperature. It should be fluid but not warm.

In a separate bowl, whip the chilled heavy cream and powdered sugar until soft peaks form. Be careful not to overwhip—it should be light and airy.

Gently fold the cooled chocolate mixture into the whipped cream using a spatula. Work slowly and carefully to preserve the mousse’s texture.

Spread the mousse evenly over the cooled brownie layer. Refrigerate for at least 2 hours, or until the mousse is firm and set.

Step 3: Top with Ganache
Place the chopped chocolate in a heatproof bowl. Heat the heavy cream until simmering and pour it over the chocolate.

Let it sit for 2–3 minutes without stirring, then whisk until smooth and glossy.

Pour the ganache over the mousse layer and spread it evenly with an offset spatula. Refrigerate for another hour, or until the ganache is set.

Once fully chilled, slice with a sharp knife. For clean edges, wipe the knife between cuts.