Mix the flour and cornflour with the eggs, then add all the remaining ingredients except the beaten egg yolk. Work rapidly with your hands without over-kneading, then leave the dough to rest in the refrigerator for 1 hour.
Preheat the oven to 200°C (400°F), Gas Mark 6.
Remove the dough and roll out. Cut out the biscuits with a 2.5 cm (1 inch) pastry cutter. Place on a greased baking tray and glaze the tops with the egg yolk. Bake for 10 minutes.