Stir together cold water and cornstarch until dissolved. Whisk cornstarch mixture, sweetened condensed milk, eggs, and vanilla into cooled chocolate mixture. Bring to a boil over medium heat, stirring constantly. Boil 1 minute or until mixture thickens. Do not overcook.
Remove from heat and stir in butter. Pour into pie crust. Cover and chill for at least 8 hours, or overnight. Top with whipped cream to serve
Adapted from Southern Living