Cream together butter and sugar until fluffy. Add egg yolks, vanilla and salt; beat well. Gradually add sifted all-purpose flour, mixing well.

Shape into 3/4 inch balls; dip in a slightly beaten egg whites, then roll in 3/4 cup finely chopped walnuts. Place 1 inch apart on greased cookie sheet. Press down center of each with thumb.

Bake at 350 degrees for 15 to 17 minutes. Cool slightly; remove from pan and cool on rack. Just before serving, use 1/3 cup cherry or strawberry preserves to fill centers of cookies. Makes 3 dozen.