Pour milk into a 3-quart saucepan over medium heat. Heat just until milk begins to bubble around the edges. Do not boil. Remove from heat and whisk in chocolate chips until melted. Cool slightly. Stir cornstarch into cold water until dissolved. Whisk cornstarch mixture, sweetened condensed milk, eggs and vanilla into chocolate mixture. Bring to a boil over medium heat, whisking constantly. Boil one minute or until mixture thickens and is smooth; do not overcook. Remove from heat and whisk in butter. Spoon mixture into pie crust. Cover and chill 8 hours. Top with whipped topping and candies.