Wash the apples, peel and core them. Cut into smaller pieces. Dip them in the apple sauce, cider, vanilla and sugar and let them soak 10-15 minutes.
Finely chop the hazelnuts and mix them with butter and sugar until creamy.
Add the eggs and the egg whites and finally the flour. The dough should be rather thin and batter-like. Pour it into a greased cake pan. Let the apple pieces drip dry (through a cloth) and distribute them onto the dough.
Bake the apple cake at 180°C for 45-50 minutes. If the cake quickly browns on top, cover it with aluminum foil for the rest of the baking period. When finished, let cool.
Pour the liquid from the apples into a pot, boil and reduce it to a thick syrup. Pour the syrup over the cake. Top the cake with lightly whipped cream.