Preheat oven to 180 C / 350 F and line a baking sheet with baking paper or a silicone baking mat

Cream together butter and sugar in a medium sized mixing bowl until light and fluffy.

Add the peanut butter and egg yolk and stir thoroughly to combine.

Sift in the flour to make a soft dough and then fold in the chocolate.

Pinch off walnut sized bits of dough and roll into balls.

Place the dough balls on the baking sheet and press down with the tines of a fork until slightly flattened.

Bake for 10-12 minutes until lightly golden. Leave to cool on the baking tray for a few minutes before transferring to a wire rack to cool completely.