Sift flour, sugar, cocoa, baking powder and salt into a large bowl. Rub in the butter using your fingers. Add the egg and milk. Chill the dough in the freezer for about 20 minutes until it’s firm enough to handle. Preheat oven to 200 C and lightly grease two baking trays. Take a scoop of dough with a teaspoon and roll into a ball. Roll the ball in icing sugar, then place on the prepared baking sheet, leaving room for spreading. Continue until all the dough is used up. Bake for 8 – 10 minutes or until just set. Leave to cool before transferring on to a wire rack. These are best within minutes of coming out of the oven, but they will keep for a day or two in an airtight container
Top Tip for getting the striking pattern on top of the cookies: work quickly when you are rolling the dough in the icing sugar as the dough needs to be very cold to ensure you get the lovely pattern of icing sugar on top. Also get each batch in the oven as quickly as possible and make sure your oven is very hot. You can make the dough up to 1 month ahead of time and keep it in the freezer. Remove from thre freezer 10 minutes before required and bake as per the recipe.