½ cup butter or margarine, softened
2/3 cup sugar
1 egg, separated
2 Tbs. milk
1 tsp. vanilla
1 cup flour
1/3 cup cocoa
¼ tsp. salt
1 cup finely chopped walnuts

Filling:
½ cup powdered sugar
1 Tbs. butter or margarine
2 tsp. milk
¼ tsp. vanilla
26 milk chocolate kisses, unwrapped

Beat butter, sugar, egg yolk, milk and vanilla until light and fluffy.  Combine flour , cocoa, and salt; gradually add to creamed mixture.  Cover and chill 1 hour. 
Lightly beat egg white.  Shape dough into 1-inch balls; dip in egg white, then roll in nuts.  Place on greased baking sheet.  Make and indentation with thumb in the center of each cookie.  Bake at 350 for 10-12 minutes. 
Combine filling ingredients until smooth; Spoon ¼ tsp. into each warm cookie, gently press chocolate kiss in center.  Cool on wire racks.