Preheat the oven to 180° and lightly butter a baking tray.
Cream together butter and sugar until they are light and fluffy. Stir in flour, cocoa powder and orange zest (grated finely). Divide dough into small balls and place on the baking tray. Lightly press each ball with the back of a fork.
Bake for 10 - 15 minutes until firm around the edges. Cool on a wire rack.