In a medium bowl, beat butter and sugar with an electric mixer until fluffy. Beat in egg yolk, vanilla and salt. Gradually beat in flour until blended. Wrap dough and chill at least for 1 hour. Heat oven to 325°. Have ungreased cookie sheets ready. Lightly beat egg white in a small bowl. Shape rounded teaspoonfuls of dough into balls. Dip in egg white, and then roll in peanuts. Place 2 inches apart on an ungreased cookie sheet. Bake 12-14 minutes until lightly golden. Remove to wire rack to cool. Store in an airtight container at room temperature for up to three weeks. Makes 22.