Combine chocolate and milk in heavy large saucepan on low heat, stirring until chocolate melts. Add sugar and corn syrup; increase heat to medium. Bring mixture to a boil, stirring constantly. Boil, without stirring, until candy thermometer reaches 238ºF or soft-ball stage (when 1/2 tsp. syrup dropped into very cold water forms soft ball that flattens when removed from water), about 8 min.