COOK water and chocolate in medium saucepan on medium-low heat 3 to 4 min. or until chocolate is completely melted and mixture is well blended, stirring constantly with whisk. Add sugar; mix well.
BRING to boil on medium-high heat. Cook 3 min., stirring constantly. Gradually add milk, stirring with whisk until well blended.
STIR in vanilla; cook on medium heat 2 to 3 min. or until heated through, stirring occasionally.