Turn on the oven at 180 degrees.
Put the butter, sugar and vanilla sugar in the KitchenAid and mix it until it gets fluffy. Add the egg yolks one by one and keep mixing.
Meanwhile, beat the egg whites stiff.
Mingle the floor, almond powder, baking powder and the yoghurt in a bowl and then add the mixture into the KitchenAid. Gently fold the egg white into the batter.
Peel the apples and slice 2 of them into little cubes. Add them to the batter and stir. Slice the last apple into parts.
Put the batter into a buttered cake mold and finish with the apple in parts on top of the cake.
Put the cake in the oven for approx. 50 minutes.
TIP: eat it while it's still lukewarm.
Buon Appetito!