Chop 150 g dark chocolate and melt over a warm water bath. Beat butter, salt, sugar and vanilla sugar until creamy with the whisk of the hand mixer. Stir in the egg.
Stir in melted chocolate. Mix flour, baking powder and cocoa, stir into the butter-chocolate mixture. Coarsely chop 100 g dark and milk chocolate, fold into the dough. Line 4 baking trays with baking paper.
Use two teaspoons to place small heaps on the baking parchment (keep a little distance, biscuits run apart). Bake one after the other in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for 10-12 minutes.
Let it cool down.