Sift the flour, salt and cayenne into a bowl then rub in the butter until it resembles breadcrumbs. Add the sugar and whisked egg and mix into a stiff dough.
Turn out the dough onto a floured surface and knead gently until smooth. Wrap in plastic wrap and chill in the fridge for about 30 minutes.
Roll out on a floured surface until about 5mm thick then cut into about 30 biscuits. Place on a greased baking tray and use a fork to give some character. Place in oven at 180 degrees C for about 12-15 minutes or until golden.