Cream together the butter and the icing sugar, then gently mix in the other ingredients until it comes together. Wrap in clingfilm and chill for at least an hour.
125 g unsalted butter, 75 g icing sugar, 0.5 tsp salt, 1 egg, 0.5 tsp vanilla extract, 240 g plain flour (all-purpose flour), 20 g cocoa powder