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 3 cm balls; place about 5 cm apart on ungreased cookie sheets. Press thumb in each cookie leaving an indention. Bake at 150ºC or 20 to 25 minutes; do not brown. Cool on wire racks. Spoon about half teaspoon Chocolate Frosting or Powdered Sugar Glaze in each cookie indention. Recipe continues on Thumbprint Cookies-Part 2. ----