Cream butter and sugar till fluffy.
Add egg yolks, vanilla and salt; beat well.
Gradually add flour, mixing well.
Shape in 3/4" balls; dip in egg whites, then roll in nuts.
Place 1-inch apart on greased cookie sheet, press down centers with thumb.
Bake at 350 degrees for 15 minutes. Cool slightly; remove from pan and cool on rack.
Just before serving, fill center with preserves.