In small bowl, stir together cold water and cornstarch until dissolved. Stir cornstarch mixture, condensed milk, eggs and vanilla into chocolate mixture. Heat to boiling over medium heat, stirring constantly with whisk. Boil 1 minute or until mixture thickens and coats the back of a metal spoon. Remove from heat; stir in butter. Spoon into crust. Cover; refrigerate at least 8 hours.