Sift 375g flour and a pinch of salt into a bowl.
Rub in 225g diced butter using fingertips until the mixture resembles fine breadcrumbs.
Stir in the cheeses, mustard powder, cayenne, and two egg yolks.
Mix in 4 to 5 tablespoons of cold water to form a firm dough.
Wrap the dough in cling film and chill in the fridge for 30 minutes.
Preheat the oven to 190°C and line a baking sheet with parchment.
Roll the dough into a square roughly the thickness of a coin.
Cut the square in half and then into 1cm strips.
Bake for 10-15 minutes until crisp and allow to cool on the tray.