Blend butter, egg yolk, vanilla, and sugar. Blend flour and salt into the mixture. Refrigerate for one hour. Lightly beat egg whites. Roll the dough into balls about the size of a golf ball. Dip the balls into egg whites. Roll into chopped nuts. Place on a lightly greased cookie sheet. Press down on the middle of the ball with your thumb. Bake for 10-12 minutes. Place on cooling rack. Fill with caramel, frosting, lemon curd, or jam.