In bowl mix flour, sugar and butter together, either by hand or in electric mixer. Add egg yolk and knead in well. Divide mixture into 4 parts and roll each into square or circle about 1 cm thick. Mark pattern in crust by lightly tapping with tip of fork. Cut each circle into 8 triangles or each square into 8 smaller squares. Place pieces on lightly buttered and floured baking sheet. Bake at 350 degrees 15 minutes, then reduce heat to 300 degrees and bake 30 minutes more, until shortbread is delicate light brown. Makes 32 shortbreads.