Put the butter and sugar into a bowl and beat together until pale and creamy. Beat in the egg, then stir in the flour, cocoa powder, bicarbonate of soda and chocolate chunks.
Heap 8–9 rough-textured spoonfuls of the batter on to each prepared baking sheet, spacing them well apart so they have room to spread. Bake in a preheated oven, 180˚C (350˚F), Gas Mark 4, for about 10 minutes, until slightly firm around the edges. Transfer to a wire rack to cool slightly before serving.