Step 1: Make the Brownie Base
Preheat your oven to 350°F (175°C). Line an 8×8-inch baking pan with parchment paper, leaving overhang for easy lifting.

In a mixing bowl, whisk together melted butter, sugar, eggs, and vanilla until smooth.

Sift in cocoa powder, flour, salt, and baking powder. Fold everything together gently until just combined.

Pour the batter into the prepared pan and spread evenly. Bake for 20–25 minutes or until a toothpick comes out with moist crumbs.

Let the brownie cool completely in the pan before moving to the next step.

Step 2: Prepare the Chocolate Mousse
Heat milk in a small saucepan until just steaming. Pour it over the chopped chocolate in a heatproof bowl. Let it sit for 2 minutes, then stir until smooth and let it cool to room temperature.

In another bowl, whip the heavy cream and powdered sugar to soft peaks. This means the cream should hold its shape but still be soft.

Gently fold the cooled chocolate mixture into the whipped cream. Use a spatula and work slowly to keep the mousse airy.

Spread the mousse evenly over the cooled brownie base. Refrigerate for at least 2 hours until set.

Step 3: Make the Ganache Topping
Place the chopped chocolate in a bowl. Heat the heavy cream in a small saucepan until simmering, then pour it over the chocolate.

Let it sit for 2–3 minutes, then stir until smooth and glossy.

Pour the ganache over the set mousse and spread it with an offset spatula. Refrigerate for another hour until firm.

Once chilled, lift out of the pan and slice with a warm knife for clean edges.