Heat milk until it just begins to bubble around the edges in a 3-quart saucepan over medium heat (do not boil). Remove from heat, and whisk in chocolate morsels until melted. Cool slightly.
Stir together cold water and cornstarch until dissolved.
Whisk cornstarch mixture, sweetened condensed milk, eggs, and vanilla into chocolate mixture. Bring to a boil over medium heat, whisking constantly. Boil 1 minute or until mixture thickens and is smooth. (Do not overcook.)
Remove from heat, and whisk in butter. Spoon mixture into piecrust. Cover and chill at least 8 hours.
Beat whipping cream at high speed with an electric mixer until foamy; gradually add sugar, beating until soft peaks form. Spread whipped cream evenly over pie filling, and sprinkle with pecans and candy bar pieces.