combine chocolate and 2 cups of milk in medium saucepan; cook over medium heat, stirring constantly, just until mixture boils. remove from heat and set aside.

stir together flour, sugar, cornstarch and salt in medium bowl. whisk remaining 1 cup milk into egg yolks in seperate bowl; stir into sugar mixture. gradually add to chocolate mixture. cook over medium heat, whisking constantly, until mixture boils; boil and stir 1 minute. remove from heat, stir in butter and vanilla

pour I to prepared pie crust, press plastic wrap directly into surface. cool; refrigerate until well chilled. top with whipped cream, if desired.