Cream fat, sugar, vanillin sugar and salt with the whisk of the hand mixer. Alternately stir in eggs and 2 tablespoons of flour. Mix remaining flour, ground hazelnuts and baking powder.
Briefly stir in 75 g soured milk and flour mixture. Wash, peel, core and dice the apple. Fold apple cubes into the dough. Place 2 paper cups in each of the 8 holes of a muffin tin (1 hole contains approx. 110 ml) and distribute the dough evenly in the cups.
Bake in the preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for about 20 minutes. Let the finished muffins cool down on a cake rack. Mix 75 g soured milk and put a blob on each muffin.
Sprinkle with hazelnut leaves and decorate with a fresh, thin slice of apple just before serving.