MELT butter in large saucepan on low heat. Add marshmallows, caramel bits and water; cook 5 min. or until marshmallows are completely melted and mixture is well blended, stirring constantly. Remove from heat. Add cereal; mix well.
PRESS firmly onto bottom of 13x9-inch pan sprayed with cooking spray; cool completely.
MELT chocolate as directed on package; drizzle over dessert. Let stand until firm before cutting dessert into bars.