Preheat oven to 350 degrees F (175 degrees C). In a saucepan, whisk together 2 cups sugar, cocoa and flour. Blend in evaporated milk and vanilla. Beat egg yolks, and stir into pan. Add the butter or margarine. Heat, stirring constantly just until butter is melted. Pour filling into unbaked pie shell. Bake in preheated oven for 35 to 40 minutes, or until pie is not "wobbly" when shaken.
Beat egg whites until soft peaks form. Gradually add 1/4 cup sugar, beating constantly, until stiff peaks form. Spread meringue on pie. Return pie to oven, and bake until meringue is golden.