In a large mixing bowl, beat butter until soft.  Add half of the flour, all of the sugar, egg yolks, and vanilla.  Beat until combined.  Beat in the remaining flour.  Cover and chill at least one (1) hour.

Shape dough into balls.  Roll dough into egg whites and then into the nuts.  Place on a greased cookie sheet about apart.  Make an indent in the dough with your thumb.

Bake at 37F for 10-12 minutes.

Fill the centers with your favorite jam when the cookies have cooled.