Combine 1 1/4 cups sugar into the cocoa powder, cornstarch & salt in a heavy saucepan. Whisk to remove any lumps. Gradually whisk in the milk. Cook over medium heat, stirring constantly, until the mixture thickens & comes to a boil. Boil 1 minute, remove from the heat. Beat the egg yolks until thick & lemon colored. Stir 1/4 of the hot mixture into the yolks; add the yolks to the remaining hot mixture, stirring constantly. Cook over medium heat for 2 minutes. Remove from the heat & stir in the butter & vanilla. Pour into the pastry shell. Preheat oven to 400°. Combine the egg whites & cream of tartar. Beat until foamy. Gradually add the remaining 6 TBL sugar, 1 TBL at a time, beating until stiff peaks form. Spread over the filling, sealing to the edge of the pastry. Bake 8 minutes or until lightly browned. Cool before serving.
