Begin by chopping the pistachios in a food processor, pulse until you have a fairly fine meal. Next add the flour, grated parmesan, diced butter, egg yolk and cayenne. Pulse until a dough forms. Do not over mix.
Remove the dough from the machine and bring it together. Roll into a long sausage shape. Wrap in clingfilm and place in the fridge for an hour.
Pre-heat the oven 175C fan. Slice the desired amount and place on a baking tray. Bake for 12-14 minutes or until lightly golden brown.
Cool on a wire rack, store in an airtight container for up to one week.
Notes
For a Gluten-free version substitute the strong white flour with Doves Farm Gluten free plain white flour.