Preheat the oven to 160°c/Gas 3. Line 2 baking trays with greaseproof paper and set aside.
Beat together the butter in a bowl with the sifted icing sugar until the mixture is smooth.
Add the flour and cinnamon and flour, rub together using your fingers.
Stir in the egg yolk and milk, add the cranberries. Shape into a ball and chill for 30 mins.
Roll out the dough on a lightly floured surface to a thickness of about ½". Cut out the shortbreads using a small star shaped biscuit cutter. Reshape any remaining dough into a ball and repeat until all the dough is used up.
Place the biscuits onto the baking trays and bake in the oven for 10-12 mins, until they are beginning to brown.
Leave to cool on a wire rack and store in an airtight tin.