1. Heat oven to 350°F. Beat butter, sugar and vanilla in large bowl until creamy. Stir together flour and cocoa; gradually add to butter mixture, beating until blended. Mold scant tablespoon dough around each chocolate piece, covering completely. Shape into balls. Place on ungreased cookie sheet.
2. Bake 8 to 10 minutes or until set. Cool about 1 minute; remove from cookie sheet to wire rack. Cool completely. Roll in powdered sugar. Roll in sugar again just before serving, if desired. Makes about 48 cookies.