Sift flour into a large bowl; dot with butter.
Make a well in center of flour; add granulated sugar, 5 egg yolks, salt and lemon peel.
Knead ingredients to make a smooth dough.
Place in a pastry bag fitted with a plain nozzle.
Pipe S shapes at equal intervals onto greased baking sheets.
Refrigerate 30 minutes.
Preheat oven to 375°F (190°C).
Beat remaining egg yolk and brush over cookies.
While egg yolk on cookies is still moist, sprinkle with sugar crystals.
If any sugar falls onto baking sheets, remove with a damp pastry brush to avoid burning.
Bake 8 to 10 minutes or until golden.
Cool on baking sheets about 5 minutes.
Remove from baking sheets with a spatula; cool completely on a rack.