Grease a springform pan (26 cm Ø) with a tube sheet insert and dust with flour. Peel and halve apples, remove core. Sprinkle with lemon juice and set aside. Cream fat, sugar, vanilla sugar and salt with the whisk of the hand mixer. Stir in the eggs one after the other.
Mix flour and baking powder, stir in. Halve the dough. Mix ground nuts and cinnamon, stir into one half of the dough with milk. Put half of the nut dough into the form. Add half of the light-coloured dough. Do the same with the remaining dough. Place the apples on the dough with the round side down. Bake in the preheated oven (electric range 175 °C/ convection oven: 150 °C/ gas: level 2) for 45-50 minutes. Let cool on a cake rack, remove from the tin. Roast the hazelnut flakes in a pan without fat until golden brown, remove.
Place the apples on the dough with the round side down. Bake in the preheated oven (electric range 175 °C/ convection oven: 150 °C/ gas: level 2) for 45-50 minutes. Let cool on a cake rack, remove from the tin. Roast the hazelnut flakes in a pan without fat until golden brown, remove. Stir icing sugar with Calvados until smooth. Spread the cake with it. Sprinkle with hazelnut leaves and let dry