Thumb Print Cookies

2/3 cup butter
1/3 cup sugar
2 egg yolks (retain whites, slightly beaten)
1 tsp vanilla
1/2 tsp. salt
1 1/2 cups sifted flour
3/4 cups finely chopped nuts (optional) 

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

Shape into 3/4-inch balls, dip in egg whites and roll in nuts.  Place
1 inch apart on greased cookie sheet.  Press down center of each with
your thumb.

Bake at 350F for 15 to 17 minutes.  Cool slightly, then remove from pan
and cool completely on rack.  Before serving, fill centers of cookies 
with your favorite jam.