Chop 120 g chocolate and melt in a bowl over a warm water bath. Beat butter and sugar with the whisks of the hand blender for about 8 minutes until creamy. After 4 minutes add egg and vanilla flavour.
Stir in melted chocolate. Mix baking soda, flour, salt and cocoa. Stir into the butter and egg mixture. Coarsely chop 280 g chocolate and fold into the dough. Place 8 biscuit heaps with plenty of space between them on 2 baking trays lined with baking paper.
Bake the trays one after the other in a preheated oven (electric cooker: 175 °C/ convection oven: 150 °C/ gas: level 2) for approx. 12 minutes.