Whip the egg yolks with 100 g powdered sugar. Beat the egg whites with 200 g of the remaining sugar. Mix gently the two compounds with the cocoa and the potato starch to incorporate all the ingredients. Prepare a baking sheet and put the mixture on it; level off with a spatula in a 5 mm layer.
Preheat the hoven to 200°C and bake the marquise for 8/9 minutes. Dust the marquise with some sugar. Line a mold with the marquise.
For the dark chocolate mousse, whip the cream and the egg yolks. Soak gelatin in cold water and heat it in microwave with some milk or cream to dissolve it completely. Melt the chocolate in a bowl set over a pan of simmering water (no more than 50°C). Add some gelatin, egg yolks and cream and whip it. When it’s ready, fill 1/3 of the mold with the dark chocolate mousse, level it and set in freezer.
Repeat process with milk chocolate and then with the white one. To guarantee a good set, make it freeze for at least 2 hours. Decorate with three chocolate curls.