Dice butter for the crumbles. Put 150 g flour, 75 g sugar, 1 pinch of salt, vanilla sugar, egg yolk and butter in a bowl and work into crumbles with your hands. Chill for about 30 minutes.
Meanwhile mix 250 g flour, cocoa, 125 g sugar, baking powder and baking soda in a bowl. Put the egg, oil and milk into a high mixing bowl and mix with the whisk of the hand mixer. Stir the milk mixture with the whisk of the hand mixer into the flour-cocoa mixture. Line the hollows of a muffin tray (12 hollows) with baking cups. Spread the dough evenly in them. Place one ball of hazelnut pastry in each centre of the trough. Sprinkle the crumbles evenly over them. Bake in the preheated oven (electric range: 175 °C/ convection oven: 150 °C/ gas: level 2) on the middle shelf for about 20 minutes.
Remove, place on a cake rack, let cool a little. Lift out of the troughs and allow to cool on the grid.