Cream butter; gradually add sugar, beating at medium speed of an electric mixer until light and fluffy. Add egg yolks, one at a time, beating well after each addition. Stir in vanilla. Combine flour and salt; add to creamed mixture, mixing well. Chill dough at least 1 hour. Shape dough into 1-inch balls; place about 2 inches apart on ungreased cookie sheets. Press thumb in each cookie leaving an indention. Bake at 300 degrees for 20 to 25 minutes; do not brown. Cool on wire racks. Spoon about 1/2 teaspoon Chocolate Frosting or Powdered Sugar Glaze in each cookie indention. Recipe continues on Thumbprint Cookies-Part 2. ----