Chop 150 g white chocolate into small pieces. Cream butter, sugar, salt and vanilla sugar with the whisk of the hand mixer until the sugar is dissolved. Stir in the egg. Mix 220 g flour and baking powder and stir in. Add chopped chocolate. Halve the dough. Stir under one half cocoa, under the other half 1 tablespoon flour.
Using one tablespoon, place 10 piles of dough on 2 baking trays lined with baking paper. Bake the trays one after the other in a preheated oven (electric cooker: 200 °C/ convection oven: 175 °C/ gas: level 3) for 10-12 minutes. Remove from the baking tray, place on a rack and allow to cool. Chop 30 g white chocolate and dark chocolate separately and melt each in a small bowl over a warm water bath. Pour the chocolate into 2 piping bags or freezer bags and cut off a small corner. Pour chocolate over the cold cookies and chill for about 20 minutes
1 hour waiting time