Combine sugar, cornstarch and salt in medium saucepan, slowly whisk in milk until smooth. Heat over medium heat just to a simmer, remove fron heat and scrape all sides, whisk in yolks whisking at all time, return to heat when it just starts to simmer cook 1 minute, remove from heat, whisk in butter, vanilla and chopped chocolate, stir until choclate melts.Cool in bowl, stirring to prevent a crust from forming or becoming set, just until room temperature. Remove plastic wrap from vanilla layer of pie and pour and carefully cover chilled vanilla cream. Chill 8 hours or best overnight. Top with whipped cream before serving