Stir butter and sugar until soft and fluffy. Add one egg at the time while stirring. Add vanilla powder.

Mix potato starch, flour and baking powder. Gently add it to the batter and blend well. Pour in a spring form with funnel inset. Bake for about 34 minutes on the bottom of the oven, 175°C.

Leave in the form to cool for a few minutes and turn it over on a plate.