1Whisk together 2/3 cup sugar, flour, cocoa, and egg yolks in a saucepan. Stir in milk. Cook over medium heat, stirring constantly until thick. I start with a whisk and then use a spoon to stir the custard as it cooks.
2Remove pan from heat, and add vanilla and butter or margarine. Mix well, and pour filling into a baked, deep dish pie crust.
3In a mixing bowl, beat the egg whites until peaks form. Slowly add 1/2 cup sugar, and beat again until stiff peaks form. Spread on pie.
4Bake at 350 degrees F (175 degrees C) for 10 to 12 minutes, or until meringue browns. Let pie cool several hours before serving.