To make the pastry, put the flour, cocoa, icing sugar and 1/2 tsp salt in a food processor and pulse briefly until combined. Add the butter and pulse for a few seconds at a time until the butter has disappeared into the flour, creating a sandy texture. Add ice-cold water, 1 tbsp at a time, pulsing in between until the pastry comes together in clumps. Don’t over-process or the pastry will become tough. Tip onto a sheet of clingfilm and draw up the sides, pressing the pastry gently into a ball. Wrap and chill for 30 minutes. The pastry also freezes brilliantly for up to 3 months if you want to make ahead.