1Make your meringue first.
Put the egg whites in a medium-sized mixing bowl. Add the cream of tartar. Beat the egg whites at high speed with an electric mixer until soft peaks form. With mixer running, gradually add the sugar, one tablespoon at a time, and beat until stiff peaks form. Fold in the vanilla. Set to the side until filling is made.
2Combine sugar, cornstarch, milk, and cocoa in 2 quart saucepan. Stirring constantly, cook over medium heat until mixture begins to bubble. Continue stirring for 2 minutes.
3Mix a little of the hot mixture into the egg yolks, beating rapidly to avoid cooking the yolks. Stir the warm yolk mixture into the remainder of the chocolate mixture, and cook for an additional 90 seconds or until thickened.
4Remove from heat, and stir in butter and vanilla. Pour filling into pie shell and top with meringue.
5Bake at 375 degrees until meringue is lightly browned.