50g/1¾ oz finely grated parmesan, plus a little extra
1 egg, beaten
Place the butter and flour into the bowl of a food processor or Kenwood with the salt, cayenne, mustard powder and cheeses. Process together to begin with, and then finely pulse the mixture in short spurts as you notice the mixture coming together – it will eventually bind without the need for egg or water. Wrap in cling film and leave to chill in the fridge for at least 30 minutes.
Preheat the oven to 180C/350F/Gas 4.
Lightly flour a work surface and gently roll out the pastry to about the thickness of two pound coins. Cut out the biscuits to the size and shape you wish: about 3cm round makes approx. 2 dozen.Alternatively, roll into a sausage, wrap in cling film and chill then slice into rounds to bake.The sausage can be frozen then partially defrosted before slicing.
Lay them out on a greased baking tray about 2cm apart.Carefully brush the surfaces of each bite with the egg and sprinkle over a little finely grated parmesan. Bake for 10 minutes, until they are golden-brown in colour.
Carefully lift the bite off the tray using a palette knife and place on a rack to cool.The biscuits will keep well in a sealed container for a few days if they get the chance!