Peel and quarter the apple and cut out the core. Cut the apple quarters crosswise into fine slices. Sprinkle with lemon juice to prevent them from turning brown. For the dough, mix fat, sugar, vanilla sugar, salt, lemon peel and rum with the whisk of the hand mixer until foamy.
Stir in the eggs one by one. Mix flour and baking powder and also stir in. Fold apple pieces and chocolate drops into the dough. Pour the dough into a greased box form (20 cm long).
Bake in the preheated oven (electric: 175 °C/ gas: level 2) for 60-70 minutes. Then let the cake cool down on a cake rack. Dust with icing sugar before serving. Walnut ice cream tastes good with it.
Results in about 15 slices.