Preheat oven to 350 degrees. With an electric mixer, cream together butter & sugar. Add egg yolks & vanilla and mix well. Sift flour and salt together and beat into butter mixture until well mixed. When ready to bake, use an ungreased cookie sheet and place cookies 1" apart. Bake for about 10 minutes, but DO NOT brown them. Remove cookies from cookie sheet while still warm and cool on sheets of waxed paper. Decorate if you like.