Peel and quarter apples, remove core and cut apple quarters into small pieces.
Separate 5 eggs; beat the egg whites and 3 tbsp. cold water until stiff. Finally, pour in the sugar. Fold in 8 egg yolks individually.
Mix flour and starch, sieve onto the egg foam and carefully fold in. Carefully fold in apple pieces and place in a greased, flour-sprinkled ring cake mould (2.5 litres capacity; 22 cm Ø). Bake in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: see manufacturer) for approx. 40 minutes (test with sticks).
Take the cake out of the oven. After about 10 minutes, carefully turn out of the tin and let it cool down on a cake rack. Dust with icing sugar and cocoa. Serve with sour cream.