1. Rub the butter into the flour and salt, and then add the egg yolk, 60g cheese and enough cold water 2-3 tablespoons to form a stiff dough. Chill for 30 minutes.
2. Form the dough into an oblong and roll out very thinly onto a board.
3. Sprinkle some cheese over two thirds of the dough, bring the unsprinkled part over half the sprinkled part in such a way as to trap air then bring the remaining two third over to make three layers. Press the open edges together lightly. Turn with the folds to the left hand side.
4. Roll out again into an oblong and repeat the sprinkling of the cheese and folding of the dough. Roll out to 1/2 inch 10mm thick and trim the edges.
5. Cut into strips 2 inches long and then into wafers 1/2 inch 10mm wide. Place on a baking sheet, chill for 20 minutes and bake for 6-7 minutes in the hottest part of the oven at 220°C 425°F gas 7.