Coarsely chop the chocolate and melt in a heatproof bowl set over a pan of simmering water, let cool to room temperature. In the bowl of an electric mixer, beat the butter with the sugar, vanilla sugar and salt until fluffy. Beat in the egg and the melted chocolate. Mix in the flour, cocoa, baking powder and milk. The batter should not be too liquid, if necessary, add a little more flour. Cover and refrigerate 2 hours.
Preheat the oven to 180°C (approximately 350°F) convection. Line 2 baking sheets with parchment paper.
Break off 2 cm (approximately 3/4-inch) in diameter pieces of dough and roll into balls. Press flat, roll the top in powdered sugar and place on the baking sheets. Bake until the cookies are crisp on the outside, but still soft inside, about 10 minutes. Cool on a wire rack.