Mix flour and butter with a pastry blender until the mixture forms crumbs. The butter particles should be about the size of green peas.
Add 4 egg yolks, the sugar and salt. Work the mixture together speedily, then knead for a minute or so. Roll the dough in a ball, cover with wax paper, and place in refrigerator overnight.
Preheat oven to 350°F. Roll out dough between 2pieces of wax paper into a sheet 1inch thick. Cut into rounds with a 2½-inch cookie cutter.
Beat the whole egg. With a knife, score a latticework pattern on top of each biscuit. Brush egg wash on top of biscuits.
Place biscuits on a cookie sheet, and bake in the preheated oven for 25 to 30 minutes, or until golden brown and firm in the middle.
Let cool completely before eating. The texture should be crumbly, almost like a shortbread.
These will last in a cookie jar for weeks, in the refrigerator for months and in the freezer, if properly protected, probably indefinitely. After storage, gently warm them in a 200°F. oven before serving.