100g/3½ oz plain flour, plus extra for flouring
pinch cayenne pepper
1 heaped tsp mustard powder
pinch salt
100g/3½ oz cold unsalted butter, cut into chunks
50g/1¾ oz finely grated mature cheddar
1 egg, beaten
50g/1¾ oz finely grated parmesan, or similar vegetarian hard cheese, plus a little extra