Preheat oven to 350 degrees. Grease cookie sheets or line with parchment paper.
In a bowl, combine butter, brown sugar, egg yolks, and vanilla and mix well using a mixer. Add flour and blend until dough forms.
Shape dough into 3 dozen balls. Dip each ball into beaten egg white, then into chopped nuts. Place on cookie sheet and place your thump into center of cookie to make an indentation.
Bake in preheated oven for 10 to 15 minutes or until light brown in color. Remove cookies to cooling rack. Cool completely.
To make butter cream icing, beat butter, Crisco, powdered sugar, vanilla and enough milk to make the icing the consistency you like. (You may not need a full Tbs of milk.) You may add food coloring for special holiday effect.
Fill the indentation of completely cooled cookies with butter cream. Makes 3 dozen cookies. Enjoy!