Preheat oven to 400 degrees F. Mix together 1 cup sugar, the flour, and cocoa. Add 3 egg yolks to dry ingredients and gradually stir in milk. Heat mixture in Teflon pan or double boiler, stirring constantly to avoid sticking. Cook until thick, then add margarine and vanilla. Pour into baked shell. Making sure they are at room temperature, beat egg whites in a metal bowl until foamy. Add 1/3 cup sugar and beat mixture until stiff. Top the pie and bake 12 - 15 minutes until golden brown. Better when served warm.