Chopping chocolate. Cream fat, brown and white sugar, salt and vanilla sugar with the whisk of the hand mixer. Stir in egg first, then flour. Mix in the oat flakes and chocolate.
Line two baking trays with baking paper. Place 6 large piles on each baking parchment, press them a little flatter. Bake one after the other in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 12-16 minutes.
Let it cool down.