Beat the butter or margarine and sugar together until light and fluffy. Sift in the flour and cocoa and mix well. Form into small balls about the size of a walnut, place on a greased baking sheet and flatten with a damp fork. Bake in a preheated moderately hot oven, 190 c (375f) Gas mark 5, for 10 to 15 minutes. Transfer to a wire rack to cool.