COMBINE sweetened condensed milk, cocoa, vanilla and salt in large saucepan over medium heat; mix well.
STIR in water slowly; heat through, stirring occasionally (do not boil). Top with marshmallows, if desired.
COMBINE all ingredients except marshmallows in 2-quart glass measure. Microwave on HIGH (100% power) 8 to 10 minutes, stirring every 3 minutes. Top with marshmallows, if desired.