1 Heat the oven to 200°C (180°C in a fan oven), 400°F, gas 6 and line a cake tin with baking paper.
2 To make the cake mixture, melt the chopped chocolate in a bain-marie. Mix together the egg yolks, sugar, vanilla sugar and a pinch of lemon zest.
3 Beat the egg whites until stiff, place on top of the egg yolk mixture, sieve the flour, corn flour and the baking powder on top and fold in carefully so that a light, airy mixture is created.
4 Gradually fold in the melted chocolate then pour the mixture into the prepared cake tin and bake for around 30 minutes. Test with a wooden stick to see if it is done.
5 Knock out onto a wire rack, leave to cool, then cut in half horizontally.
6 To make the mousse, melt the remaining chocolate in a bain-marie and stir in the cream.
7 In another bain-marie, mix together the egg yolks and the sugar until foamy. Slowly pour the melted chocolate into the egg yolk mixture and stir in. Chill for at least 30 minutes.
8 Line a cake tin with baking paper and place one half of the cake inside it. Spread with a 1/3 of the chocolate mousse and place the other half on top. Spread another third of the cream on top and chill for another hour.
9 Remove the cake from the tin and spread the edges of the cake with the remaining chocolate mixture. Chill for 30 minutes.
10 Decorate the cake with the chocolate balls and serve in slices.