Preheat oven to 400 degrees F (200 degrees C).
To Make Chocolate Custard: In a large saucepan or double boiler combine 1 cup of sugar, cocoa powder, flour, and salt.
Mix well, then stir in egg yolks and milk.
Whisk until all ingredients are thoroughly combined. Cook over medium heat, stirring constantly, until mixture has the consistency of pudding.
Stir vanilla extract and butter or margarine into custard.
Pour mixture into baked pie shell.
To Make Meringue:
In a medium glass or metal bowl whip egg whites until foamy.
Gradually add 1/4 cup sugar, continuing to beat until whites form stiff peaks.
Spread meringue over chocolate custard, covering completely.
Bake in preheated oven for 5 to 6 minutes, until meringue is golden brown.
Chill before serving.
