Preheat the oven to 190°C/gas mark 5 and line two baking trays with greaseproof paper. Cream and mix together the butter and sugar. mix in the egg yolk and gradually put in the flour using your hands. Use your hands to mix and knead the dough until it becomes smooth and workable. Mix in the chocolate chips.
Roll the dough out on to a floured surface. Using a round cutter, stamp out as many as you can. Put these on to the lined baking trays. Bake in the oven for 10-12 minutes. Allow to cool for about 5 minutes and then transfer to a cooling wrack