Mix flour and butter with pastry blender until mixture resembles coarse crumbs. Add egg yolks, sugar, and salt, and mix together. Turn out onto a lightly floured surface and knead for a minute or so. Make 1 large ball, cover with plastic wrap, and refrigerate overnight. Preheat oven to 350 degrees F. Roll out dough between sheets of wax paper to a thickness of 1 inch. Cut into 2 1/2-inch rounds. Score a lattice-work design on each biscuit with knife. Beat whole egg and brush egg wash on top of biscuits. Bake 25 to 30 minutes, or until golden brown. Let cool completely before serving. Can be refrigerated for several weeks; warm in a 200 degrees F-oven before serving.