Beat butter at medium speed with an electric mixer until creamy; add sugar, beating well. Add egg yolks, 1 at a time, beating well after each addition; add vanilla, beating until blended.
Combine all-purpose flour and salt; gradually add to butter mixture, beating at low speed just until blended. Cover and chill 30 minutes.
Shape dough into 1-inch balls; place dough 2 inches apart on ungreased baking sheets. Press thumb into each ball, leaving an indentation.
Bake at 350° for 9 to 11 minutes or until set. (Tops of cookies will not be brown.) Remove from oven, and immediately press desired topping in center of each cookie. Remove cookies to wire racks to cool.