Mix the butter and sugar together with a wooden spoon. Stir in the flour and cocoa, followed by the chocolate chips – you’ll probably need to mix it together with your hands at this stage. Halve the dough and roll each piece into a log about 5cm thick. Wrap in cling film and chill for 1 hr or for several days. Can be frozen for up to 1 month.
Heat oven to 180C/160C fan/gas 4. Slice logs into 1cm-thick rounds, transfer to a baking tray lined with baking parchment and bake for 10-12 mins. Cool on the tray.