1. Preheat the oven to 350°F (175°C) and grease a round cake pan.
2. In a bowl, mix flour, cocoa powder, baking powder, and salt.
3. In another bowl, cream together the butter and granulated sugar until light and fluffy.
4. Add eggs one at a time, mixing well after each addition, then stir in vanilla.
5. Gradually add the dry ingredients to the wet mixture, mixing until just combined.
6. Fold in the melted dark chocolate until fully incorporated.
7. Pour the batter into the prepared cake pan and bake for 25-30 minutes.
8. Let the cake cool completely before adding the mousse layers.
9. For the dark chocolate mousse, whip 1 cup of heavy cream and fold in the melted dark chocolate.
10. Spread the dark chocolate mousse over the cooled cake.
11. Repeat the process for the milk chocolate and white chocolate mousses, layering them on top of each other.
12. Chill the cake in the refrigerator for at least 4 hours before serving.
13. Garnish with chocolate shavings or berries if desired.

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