Preheat oven to 180°C/350°F/Gas 4. Lightly grease 1-2 flat baking trays
Put the butter and sugar into a mixing bowl and beat until smooth and creamy.
Next, beat in the peanut butter. Then stir in the beaten egg and the flour. Mix until smooth.
Taking teaspoonfuls of the mixture, roll them into a ball and place them onto the baking sheet (a little apart, to allow for spreading during baking). Press down on each cookie with a fork to give nice markings when baked.
Bake for 10-12 minutes until a little golden. They will still be soft to the touch.
When baked, allow to set for about 5 minutes. Transfer to a wire tray to cool. When completely cold, store in an airtight container to keep them fresh.