Preheat the oven to 180ºC. Line two baking trays with baking paper.
Cream the butter and sugar until light and fluffy. Add the egg and beat well.
Sift the flour, baking powder, salt and cocoa together and add to the creamed mixture. Add the peanuts, roughly chopped if you wish, and mix well.
Roll tablespoonfuls of the mixture into balls, place on the prepared trays and press down with a floured fork.
Bake for 15 minutes or until firm and coloured on the base. Leave on the trays for 1–2 minutes then place on a wire rack to cool.