Beat the butter, sugar, vanilla sugar and lemon zest until frothy. Gradually add the eggs. Mix the flour with baking powder and stir in alternately with the milk. Pour the dough into a greased springform pan (26 cm).
Peel the apples, halve or quarter according to taste and cut out the casing. Score the apple pieces lengthways several times and place them in a circle on the dough.
Bake in the preheated oven at 180 ° C top / bottom heat (convection: 160 ° C) on the middle rack for about 45 minutes.
Warm the apricot jam with 1 teaspoon of water and brush the cake with it immediately after baking.