Sift together the flour, salt and cornflour, then rub in the butter with your fingertips until you have a light, crumbly mixture.
Add the egg yolk and mix lightly, adding just sufficient water to obtain a smooth, even dough, but do not overwork the pastry.
Wrap in clingfilm and chill in the refrigerator.
Using 150 g (5 oz) shortcrust pastry (you can freeze the rest), roll out to a thickness of 3 mm / 1/8 inch, and cut a disc 20 cm/8 inches in diameter.
Place on a baking sheet lined with nonstick baking parchment and prick all over with a fork.
Chill in the refrigerator.