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.
Preheat the oven to 190C/375F/Gas 5. Line a baking sheet with baking parchment. Roll out the dough to a square, roughly the thickness of a £2 coin. Cut the square in half, then cut each half into 1cm/½in strips. Transfer carefully onto the lined baking sheet and bake for 10-15 minutes, until crisp, then leave to cool on the tray.