1. CREAM together butter and sugars until light and fluffy. Whisk in egg and vanilla. Stir in sifted flour and cocoa and then half the chocolate.
2. ROLL a tablespoonful of mixture into a ball and place on a greased baking tray, pressing down slightly. Repeat with remaining mixture, allowing room for spreading. Top each cookie with remaining chocolate chunks.
3. BAKE in a moderate oven 180°C for 15 minutes or until cooked. Cool on a wire rack. Store in an airtight container until required.