Grease well a rectangular baking pan (about 1,5 litre capacity).
In a bowl combine flour, caster sugar, baking powder, cinnamon and vanilla sugar. Mix well and set aside.
Peel and chop the apples into bite sized pieces. Place apples and brown sugar in the pan and mix well with a spoon.
Cover the apples with the dry mixture and smoothen very well the top with a spoon, so that all apples are covered evenly.
Slice the butter thinly and arrange the slices close one to another on top of the dry ingredients.
Bake for about 30 minutes on 180 degrees C (350 degrees F) or until a golden brown crust is formed. Serve immediately.
Keep leftovers in airtight container in the fridge. Reheat in the microwave before serving.