In a large mixing bowl stir together flour, powdered sugar, and cornstarch. Using a pastry blender, cut in butter until mixture resembles coarse crumbs. Stir in nuts. Combine egg yolks, 1 tablespoon orange peel, and 4 tablespoons of the orange juice; add to flour mixture, stirring until moistened. If necessary, add remaining orange juice to moisten.
On a lightly floured surface, knead dough until it forms a ball. Shape dough into 1-1/4-inch balls. Arrange balls on an ungreased cookie sheet; flatten to 1/4-inch thickness by pressing with the bottom of a glass, dipping glass into granulated sugar for each round.
Bake in a 350 degree F oven for 12 to 15 minutes or until edges begin to brown. Transfer cookies to wire racks; cool completely. Frost with Orange Frosting. Garnish with additional finely shredded orange peel, if desired. Makes about 72 cookies.
In a bowl stir together sifted powdered sugar, softened butter, finely shredded orange peel, and enough orange juice to make an icing of spreading consistency.