Wash, peel and quarter the apple, cut out the core. Dice apple finely. Put the fat, salt, vanillin sugar and 150 g brown sugar into a mixing bowl and whisk with the whisk of the hand mixer until foamy. Stir in the eggs one after the other. Mix flour and baking powder and stir in alternately with 3 tablespoons of milk. Remove half of the dough and stir in the apple cubes and almonds. Stir 25 g sugar, cocoa, 2 tablespoons of milk and chocolate flakes into the remaining dough.
Line a muffin baking tray (12 troughs) with paper sleeves. Fill half of the paper sleeves with light-coloured dough. Spread dark dough on top. Bake in the preheated oven (electric cooker: 175 °C/ gas: level 2/ convection oven: 150 °C) for about 30 minutes. Remove from the oven, place on a rack and allow to cool slightly. Lift the muffins out of the cups and let them cool