Beat the butter and sugar together in a mixing bowl until pale and creamy. Stir in the egg, flour and cocoa powder and mix until smooth.
Stir in the chopped chocolate then spoon 20 mounds of the mixture on to 2 greased baking sheets, leaving space between for them to spread during cooking.
Bake in a preheated oven, 180°C (350°F), Gas Mark 4, for 8–10 minutes until lightly browned. Leave to harden for 1–2 minutes then loosen and transfer to a wire rack to cool completely. These are best eaten on the day they are made.