Beat the egg whites until stiff.
Beat the yolks, powdered sugar and vanilla sugar until foamy. Gradually add water and oil.
Sift together flour, nuts and baking powder. Stir into the egg mixture. Finally, fold in the beaten egg white.
Bake the dough in the preheated oven at 180 °C for about 50 minutes. Then allow to cool. Once the cake has cooled, cut it in half crosswise.
For the nougat cream, mix margarine and powdered sugar with yolks until fluffy, add rum and Nutella. Fill the halved cake with the cream.
Glaze the cake with chocolate icing and decorate with sugar roses to taste.