1. In a bowl over a double boiler (or in a bowl with boiling water for 2-3 minutes - as soon as the water is removed, add the egg and sugar), whisk the sugar and eggs for 1 minute, then at high speed for 4 minutes, and at low speed for 2 minutes. The mixture should form a ribbon.
2. Add the baking powder, melted butter, and quickly fold in the flour and vanilla sugar with a spatula.
3. If the mixture is not homogeneous, you can give it a quick whisk to avoid deflating the mixture.
4. Pour the batter into a greased and floured cake pan.
5. The genoise should be immediately placed in the oven.
6. Bake for 15 minutes at 180°C (160°C for an electric oven) if using a baking sheet lined with parchment paper.
7. If baking in a cake pan to later slice horizontally into multiple layers, bake for about 30 minutes.
8. Immediately remove from the pan after taking it out of the oven.
9. If using a baking sheet, it can wait a little on the sheet.
10. For the chocolate mousse:
11. Melt the chocolate with the butter in a double boiler.
12. When smooth, remove from heat and add the egg yolks. Beat the egg whites until stiff peaks form. Pour the melted chocolate over the egg whites and gently fold together to avoid deflating.
13. For assembly:
14. Fill the genoise with the chocolate mousse, no particular technique required.
15. Refrigerate overnight and it will be ready for the next day!
16. Enjoy your meal!