Preheat the oven to 170 ° C (top / bottom heat). Peel and slice the apples. Separate the eggs and beat the egg whites until stiff.
Mix the butter, sugar and vanilla sugar and egg yolks together. Slowly add flour and baking powder through the sieve. Mix all ingredients together and carefully fold in the egg whites. Add enough milk to make a light, but soft and spreadable dough.
Pour half of the batter into a springform pan and add the apples, then top up the rest of the batter. Bake the cake for 40 minutes at 170 ° C.
For the topping, roast the walnuts in a pan until they are lightly browned. Caramelize 200 g sugar in a pan, deglaze with 200 ml cream and stir until everything is dissolved.
Mix with the walnuts and pour over the cake that is still in the baking pan and let cool. Then remove the cake from the mold first.