Butter a round baking form with removable edges.
Start the oven at 175 degree Celsius.
Pour the sugar and egg in a bowl and whip it fluffy with an electrical mixer. Let the batter rest five minutes.
Add the potato flour, flour, sugar, vanilla sugar and the baking powder, and mix it together with the electrical mixer.
Pour the batter in the baking form and put the cake in the middle of the oven for 40 minutes.
Take out the cake and let it cool down in room temperature.
Pour the ingredients for the filling in a bowl and stir it together.
Make small holes all over the cake and pour a bit of the liquid on the cake. Let the liquid soak in to the cake and repeat this a few times. It might be that you can´t use all the liquid. Let the cake rest one hour.
Whip the whip cream to a hard foam, and then put it all over the cake, even on the sides.