Heat oven to 350° F. Lightly grease cookie sheet. Beat together margarine and sugar until creamy. Add egg yolks and vanilla; beat well. Add combined oats, flour and 2/3 cup ground nuts; mix well. In small bowl, beat egg whites with fork until frothy. Shape dough into 1-inch balls. Dip in egg whites; press one side into remaining nuts. Place ball nut side up 1-inch apart on prepared cookie sheet; press thumb deeply in center of each. Bake 8 minutes; remove from oven; Place one chocolate piece in each center. Return to oven and continue baking 5 to 7 minutes or until cookies are lightly browned. Remove to wire rack; cool completely. Store in tightly covered container.