Sift the flour and a pinch of salt into a bowl. Using your fingertips, rub in the butter until all the lumps are gone and the texture is like fine breadcrumbs. Stir in the cheese, mustard powder, cayenne, and egg yolks. Add 4-5 tablespoons of cold water and mix to a firm dough. Wrap in cling film and chill in the fridge for 30 minutes.