Stir water, powdered sugar, yolks and vanilla sugar thingy until creamy. Whip egg whites to very firm peaks and fold into yolk mixture with flour.
Spread the sponge mixture on a baking sheet lined with parchment paper and bake in a heated oven at 200 degrees for about 10 min.
Cream: Cream butter until fluffy, add sifted powdered sugar, softened chocolate, yolks and rum, stir until well creamy. Once cooled, cut the sponge into 4 cm wide strips, spread with cream, roll up, add the next strip and so on. Roll up until you have the size of a cake. Spread the edge and surface with cream, then sprinkle the edge and surface with chocolate sprinkles.