In a saucepan, melt butter over medium heat. Stir in corn syrup and cocoa; bring to a boil. Remove from the heat; stir in vanilla, 2 cups confectioners' sugar and milk powder. (Mixture will be stiff.)
Turn out onto a surface lightly dusted with confectioners' sugar. Knead in remaining confectioners' sugar; knead 3-4 minutes longer or until stiff. Divide into four pieces and roll each into an 18-in. rope. Cut into 3/4-in. pieces. Wrap each candy in cellophane or waxed paper. Store in refrigerator.