Combine flour, powdered sugar, and cornstarch in a large mixing bowl. Using a pastry blender or two knives, cut in 2 cups of butter until mixture resembles coarse crumbs. Stir in the 1 cup nuts. Combine egg yolks, 1 tablespoon orange peel, and 4 tablespoons of the orange juice in a small mixing bowl; add to flour mixture, stirring until moistened. If necessary, add enough of the remaining 2 tablespoons orange juice to moisten.
Knead dough on a lightly floured surface until it forms a ball. Arrange balls on an ungreased cookie sheet. Dip bottom of a fluted glass in granulated sugar, and use it to flatten one ball to 1/4-inch thickness. Repeat for remaining cookies.
Bake in the preheated oven for 12 to 15 minutes or until edges begin to brown. Transfer cookies to a wire rack; let cool.
Meanwhile in a small bowl stir together powdered sugar, 3 tablespoons butter, 1 teaspoon orange peel and enough of the 2 to 3 tablespoons juice to make sauce of speading consistency.
If desired, sprinkle with finely chopped macadamia nuts. Makes 72 cookies.