1. Preheat the oven to 350°F (175°C). Line an 8×8-inch square baking pan with parchment paper, leaving overhang for easy removal.

2. In a large bowl, whisk together the melted butter, granulated sugar, eggs, and vanilla extract until smooth.

3. Sift in the cocoa powder, flour, salt, and baking powder. Fold the mixture gently just until combined.

4. Pour the batter into the prepared pan, smooth the top, and bake for 20–25 minutes until a toothpick comes out with moist crumbs. Cool completely.

5. Heat the milk until steaming, then pour over the chopped chocolate. Let sit 2 minutes, then stir until melted and smooth. Cool to room temperature.

6. Beat the chilled heavy cream and powdered sugar until soft peaks form.

7. Add a spoonful of whipped cream to the cooled chocolate to loosen it, then gently fold in the rest until airy and fully combined.

8. Spread the mousse over the cooled brownie layer and refrigerate for at least 2 hours or until set.

9. Place the chopped chocolate in a bowl. Heat the heavy cream until simmering, pour over chocolate, and let sit 2–3 minutes. Stir until smooth and glossy.

10. Pour the ganache over the chilled mousse layer and smooth the surface. Refrigerate for 1 hour or until firm.

11. Lift the dessert from the pan using the parchment overhang. Slice with a clean knife, wiping between cuts for neat edges.