Preheat oven to 350 degrees.
In a mixing bowl beat 1/2 c butter, 2/3 c sugar, egg yolk, 2 T milk, and 1 t vanilla until light and fluffy. Combine flour, cocoa, and salt. Gradually add to creamed mixture.
Cover and chill 1 hour or until firm enough to roll into balls. In a small bowl lightly beat egg white.

Shape dough into 1 inch balls. Dip in egg white tehn roll in nuts.
Place on greased baking sheet. Make an indentation with thumb in center of each cookie. Bake for 10-12 minutes or until center is set.
Combine powdered sugar, 1 T softened butter, 2 teaspoons milk, and 1/4 t vanilla in a small bowl. Mix until smooth.
Spoon 1/4 t into each warm cookie and gently press a chocolate kiss in the center.
Carefully remove from baking sheet to wire racks to cool.