1. Beat butter, sugar, egg and vanilla in large bowl until well blended. Add flour, cocoa, baking soda and salt alternately with milk, beating until well blended. Cover; refrigerate dough about 1 hour or until firm enough to handle. Shape dough into 1-inch balls; place on ungreased cookie sheet.
2. Heat oven to 375°F. Remove wrappers from chocolate pieces.
3. Bake 8 to 10 minutes or until set. Cool 1 minute; remove from cookie sheet to wire rack. Cool completely. Spread frosting onto cookies, leaving about 1/2 inch around outer edge unfrosted; place chocolate piece in center of each cookie. Makes about 36 cookies.