100g/3陆 oz plain flour, plus extra for flouring
pinch cayenne pepper
1 heaped tsp mustard powder
pinch salt
100g/3陆 oz cold unsalted butter, cut into chunks
50g/1戮 oz finely grated mature cheddar
1 egg, beaten
50g/1戮 oz finely grated parmesan, or similar vegetarian hard cheese, plus a little extra