Preheat oven to 375 deg. F. Cream butter. Put egg yolks through sieve and add to butter. Add sugar gradually, beating until smooth. Stir in flour and almond extract. Chill. Shape into balls about 1 inch in diameter. Place on ungreased cookie sheet 2-inches apart. Press 1 almond into each ball. Bake for 12 to 15 minutes.