Cream butter, add sugar gradually and mix well. Add well beaten egg yolk, flavoring and flour.
Shape into balls about the size of a walnut.
Dip in unbeaten egg white, then in finely chopped nuts.
Dent in center with a thimble or your thumb.
Place on greased cookie sheet and bake at 350° for 5 minutes. Dent cookies again. Bake an additional 12-15 minutes.
Fill with jelly while hot.
