Beat the butter with the sugar, salt and vanilla sugar until frothy. Stir in eggs. Mix flour with baking powder and fold in. Peel, core and quarter the apple. 1 coarsely grate the apple and fold into the batter
Grease the springform pan (26 cm) and pour in the batter.
Cut into the apples several times on the curved side (lengthways and diagonally) Then place them on the dough and press lightly.
For the nut topping, mix the cold butter in flakes with the sugar and the chopped nuts. Spread the mixture on the cake. Bake the cake in the oven for about 50 minutes. Take out of the mold and let cool. Dust with powdered sugar.