Beat the butter and sugar together in a mixing bowl or a food processor until pale and creamy. Add the peanut butter, egg, flour and baking powder and stir together until combined. Stir in the peanuts.
Drop large teaspoonfuls of the mixture on to 3 large, lightly oiled baking sheets, leaving 5 cm (2 inch) gaps between each for them to spread during cooking.
Flatten the mounds slightly and bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 12 minutes until golden around the edges. Leave to cool on the baking sheets for 2 minutes then transfer to a wire rack to cool completely.