Preheat the oven at electrical heat at 250 degrees Celsius.
Pour the flour, potato flour, baking powder and the cacao powder in a bowl and mix it together.
Whip the sugar and the eggs for the cake really fluffy and white.
While still whipping pour in the dry ingredients in the sugar and egg mixture.
Place a baking paper on a deep baking sheet approx thirty by forty centimeters.
Pour the mixture evenly on the baking paper.
Place the cake in the middle of the oven for five minutes.
Take out the cake and let it rest five minutes.
Sprinkle one tablespoon of sugar on the cake.
Flip it upside down on another baking paper, and take away the paper it was cooked on. let the cake cool down.
Pour the margarine in a bowl and stir it soft with a fork.
Add the vanilla sugar and the icing sugar and stir it all to firm cream.
Add the egg yellow and mix it all one more time.
Spread the filling evenly on the cake, and roll the cake together from the short side.
Place the cake in the fridge for minimum one hour.
Before serving the cake cut it in thin slices.