Beat butter till soft. Add sugar and flour, mixing till blended evenly. Stir in yolks and vanilla, mixing until dough forms. Gather into a disk, wrap in plastic wrap and chill for 3-4 hours.
Preheat oven to 350º. When dough is chilled, roll it 1/4" thick, cut with 2" cutters and carefuly place on ungreased baking sheets. Bake for 10 – 12 minutes, till set and just beginning to turn golden at the edges. .
Cool slightly before removing to a rack to cool completely. Frost and decorate as desired with butter icing ( 1 stick butter, 4 cups sifted powdered sugar, 2-3 Tbs. cream, 1/2 teaspoons vanilla).
Adding 1 or 2 Tbs. meringue powder to the icing will result in an icing that will "crust" a bit and allow the decorated cookies to be stacked without smearing your decorations (too much).