Cream butter and sugar, add egg yolk and flour, mix thoroughly. Roll into small balls, dip in egg white and roll in walnuts. Place on cookie sheet and press the center of each one.
Cook in slow 325° oven for 5 minutes. Remove from oven and press center again. Return to oven and cook slowly for 10 to 15 minutes or until lightly brown.
Fill the hole in the top with jelly or jam while still warm.
Servings: