Cream together the butter, sugar and orange rind until light and fluffy.
Mix in the cocoa powder and flour and bring together until you have a ball of dough. Cover and chill for 10 minutes.
Shape the mixture into walnut-sized balls and place on the baking sheet, making sure they are well spaced.
Bake in a preheated oven, 190°C (375°F), Gas Mark 5, for 5 minutes. Remove the tray from the oven and lightly press down each ball of dough with your finger. Return to the oven and bake for a further 5–7 minutes, until the mixture has started to crisp on top.