In a mixing bowl, 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 or until firm enough to roll into balls. Meanwhile, in a small bowl, lightly beat egg white. Shape dough into 1 inch balls; dip in egg white, then roll in nuts. Place on greased baking sheets. Make an indentation with thumb in center of each cookie. Bake at 350 °F for 10 to 12 minutes or until center is set. Combine the first four filling ingredients in a small bowl; mix until smooth. Spoon 1/4 teaspoon into each warm cookie; gently press a chocolate kiss in the center. Carefully remove from baking sheet to wire racks to cool.