Shortbread Triangles

In large bowl, combine powdered sugar and butter. Cream until light and fluffy. Add egg yolks. Blend well. Lightly spoon flour into measuring cup and level off. Add flour and cornstarch. Stir until mixture forms a smooth dough. Shape dough into a 12 inch roll, 1-1/2 inches in diameter. Wrap in waxed paper. Press sides of roll with palm of hand to make 3 even sides forming a triangular shape. Press roll against countertop to smooth and flatten sides. Refrigerate until firm. Heat oven to 350. Slice dough into 1/4 inch slices. Place on ungreased cookie sheets. Bake 8-13 minutes until lightly browned and set. Prick tops of cookies with fork. Remove from cookie sheets to cool. If desired, dip edges in melted chocolate.