Cream butter and sugar thoroughly. Add egg yolks and vanilla, beating well, Stir in flour, mixing well.,.
Form into 3/4 -inch balls. Arrange on an ungreased baking 1 1/2 inches apart. Make an indentation in the center of each ball., Fill with the desired jam or jelly of your choice.
NOTE: Before baking, if you wish you can dip the balls in beaten egg whites, then in finely chopped nuts, add jam and bake,.