In a bowl add the butter, flour, cocoa and sugar. With your fingers rub the butter into the dry ingredients.
Add the egg and milk and with a spoon mix together and then shape the dough into a ball and leave to chill in the fridge for 30mins.
Preheat the oven to 160°c / gas 3. Line 2 baking trays with baking papers.
Roll the dough on a lightly floured surface to a thickness of about 5mm (¼inch). Cut out the biscuits using a star-shaped cutter. Knead the remaining dough back together and cut out more starts until you have used up all the dough.
Place the stars on the baking trays and bake in the oven for 13-15mins. Remove from the oven and leave to cool.
For the icing, mix the icing sugar and cocoa powder with 2-3 tbsp of water, until a thick consistency.
Spread the icing over each biscuit and then decorate with toppings.
Leave to set and then store in an airtight container.