1. Preheat the oven to 175°C (350°F) and grease a springform pan.

2. Melt the semi-sweet chocolate and butter together, then let it cool slightly.

3. In a mixing bowl, whisk eggs and sugar until light and fluffy.

4. Sift in cocoa powder and flour, then gently fold into the egg mixture.

5. Pour the batter into the springform pan and bake for 20-25 minutes or until a toothpick inserted comes out clean.

6. Let the cake cool completely in the pan before preparing the mousse layer.

7. Melt the white chocolate over a double boiler or in short bursts in the microwave.

8. Dissolve the gelatin in warm water and stir it into the melted white chocolate.

9. Whip the heavy cream until soft peaks form, then carefully fold it into the white chocolate mixture.

10. Pour the mousse over the cooled chocolate cake base and smooth the surface with an offset spatula.

11. Refrigerate the cake for at least 4 hours to allow the mousse to set.

12. Prepare the chocolate ganache by heating the heavy cream until it begins to simmer.

13. Pour the hot cream over the semi-sweet chocolate and let it sit for a minute, then stir until smooth.

14. Allow the ganache to cool slightly before pouring it over the set mousse layer.

15. Use an offset spatula to spread the ganache evenly over the cake.

16. Chill the cake for at least 30 more minutes to let the ganache firm up.

17. Carefully remove the cake from the springform pan and transfer it to a serving plate.

18. Garnish with chocolate shavings, fresh berries, or candied citrus for an elegant presentation.

19. Slice and serve chilled, enjoying the rich layers of chocolate and mousse.

20. Store leftovers in the refrigerator or wrap and freeze for longer storage.