ready in 2 h. 48 min.
Melt the chocolate in a bowl over a pan of hot, but not boiling water.
Beat together the butter, sugar, vanilla sugar and a pinch of salt until fluffy. Mix in the egg and the melted chocolate, then add the flour, cocoa, baking powder and milk and stir to incorporate. The dough should not be too runny; if necessary, add more or less milk or flour as needed. Cover and chill in the refrigerator for at least 2 hours.
Heat the oven to 200°C (180°C in a fan oven), 400°F, gas 6 and line a baking tray with grease-proof paper.
Take small portions of the dough and roll into balls. Flatten slightly, dip the top side in icing sugar and place on the baking tray. Bake in the oven for around 8 minutes, so that the biscuits are crispy on the outside but still soft inside.
Cool on a wire rack.