My Friend's Favourite Apple Cake (German)

Why I Love This Recipe
My best childhood friend loved to bake this simple apple cake. I never tried to bake it myself. 20 years later I found her recipe and finally I baked the very same cake. It's delicious and easy to make.
Submitted by: "The Rethfeldt Family"
Ingredients You'll Need
200g caster sugar
4 eggs
200g flour
200g butter, softened
1 teaspoon baking powder
1 pack vanilla sugar (24g)
about 1.5kg apple (slightly sour apples would be best, here in Hong Kong I use Gala apples)
icing sugar (for sprinkling on top the cake)
Directions
With a kitchen machine mix the butter until soft and fluffy.
Add one egg and 2 tablespoons sugar at the time until all eggs and sugar are used.
Add vanilla sugar.
Mix the flour with baking powder and once mixed together add the flour mix to the butter-egg-sugar mix.
If not soft enough add few table spoons milk.
Fill the dough into a round creased baking form of about 23cm (about 9 inch).
While preheating the oven to 180 °C (356° F) peel the apple and remove the seeds.
Cut into thin slices -about 6-8 slices per half of an apple.
Start from the middle by arranging the apple slices circular until all dough is covered.
Push apple slices slightly into the dough otherwise you have the dough on the bottom and the apple on top.
Bake until golden brown for about 50 minutes.
After cooling sprinkle with some icing sugar. It can be served hot or cold.