Step 1: Prepare the Cake Base

1. Preheat your oven to 350°F (175°C) and grease the bottom of a 9-inch springform pan. Line the bottom with parchment paper for easy removal.
2. In a small bowl, whisk together the flour, cocoa powder, baking powder, and salt.
3. In a separate bowl, beat the eggs and sugar until pale and slightly thickened, about 3 minutes. Add the melted butter and vanilla extract, mixing until smooth.
4. Gradually fold the dry ingredients into the wet ingredients until just combined. Be careful not to overmix.
5. Pour the batter into the prepared pan and spread it evenly. Bake for 10–12 minutes, or until a toothpick inserted in the center comes out clean.
6. Allow the cake to cool completely in the pan.

Step 2: Make the Dark Chocolate Mousse

7. Melt the dark chocolate in a heatproof bowl over a pot of simmering water (double boiler) or in the microwave in 20-second intervals, stirring until smooth. Let it cool slightly.
8. In a mixing bowl, whip 1/2 cup of heavy cream and powdered sugar until stiff peaks form.
9. Gently fold the whipped cream into the melted chocolate, a little at a time, until smooth and fully combined. Spread the dark chocolate mousse evenly over the cooled cake base. Chill in the fridge for 30 minutes to set.

Step 3: Make the Milk Chocolate Mousse

10. Repeat the same process as the dark chocolate mousse: Melt the milk chocolate, whip 1/2 cup of cream with powdered sugar, and fold them together. Spread the milk chocolate mousse over the set dark chocolate layer. Chill for another 30 minutes.

Step 4: Make the White Chocolate Mousse

11. Repeat the process once more for the white chocolate mousse: Melt the white chocolate, whip the cream with powdered sugar, and gently fold them together. Spread the white chocolate mousse over the milk chocolate layer. Smooth the top with an offset spatula. Chill the cake for at least 4 hours or overnight to fully set.

Step 5: Garnish and Serve

12. Once the cake is set, remove it from the springform pan and transfer it to a serving platter. Garnish with chocolate curls, cocoa powder, or fresh berries as desired.
13. Slice with a sharp knife, wiping the blade clean between cuts for neat slices. Serve chilled.