Preheat oven to 180°C/160°C fan-forced. Line 3 baking trays with baking paper.
Using an electric mixer, beat butter and sugars until light and fluffy. Add egg. Beat to combine. Sift flour, cocoa and bicarbonate of soda over butter mixture. Stir in chocolate.
Roll level tablespoons mixture into balls. Place on prepared trays, 5cm apart. Flatten slightly. Bake for 12 to 15 minutes or until light golden. Stand on trays for 5 minutes. Transfer to a wire rack to cool completely. Serve.