Mix the butter with sugar and salt until foamy. Mix the vanilla sugar with the beaten egg into the buttercream. Mix the flour and baking powder and add to the buttercream with the chopped chocolate. Knead everything into a soft dough and cover and chill for at least 2 hours. Preheat the oven to 180 ° C. Cover the baking sheet with baking paper and dust with flour. Shape the dough into walnut-sized balls and place on the baking sheet as far away as possible. Bake the cookies for about 12-15 minutes.