Cream together butter or margarine and the sugar. Add beaten egg yolk and 2 tsp lemon juice. Beat well. Add the flour and combine until well blended. Shape dough into balls which are about one inch [2-1/2 cm] in diameter. Dip balls in slightly-beaten egg white then roll in finely chopped nuts.

Place on greased baking sheet and with your finger make a fairly deep indentation in the center of each cookie.

Bake for 5 minutes and then quickly indent the centers a second time. Bake for another 10 to 12 minutes.

Fill hot cookies with brightly colored jelly or jam.