Preheat the oven to 190°C/gas mark 5 and line two baking trays with greaseproof paper. Cream together the butter and sugar. Beat in the egg yolk and gradually work in the flour using your hands. Knead very well until it becomes smooth and pliable. Mix in the chocolate chips.
Roll the dough out on to a floured surface. Using a 5cm round cutter, stamp out 28 rounds. 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 tray.