In a medium saucepan, combine sugar, flour and cocoa. Stir until mixture is well blended. Slowly pour in milk, stirring well. Slightly beat the egg yolks and gradually add to chocolate mixture; add butter. Over medium heat, cook mixture until thick, about 20 minutes, stirring often. When thick, remove from heat and stir in vanilla. Pour into pie crust. For Meringue: Beat egg whites and cream of tartar until foamy, gradually add sugar. Continue to beat until stiff peaks form. Spread Meringue over pie filling and swirl. Bake at 350 degrees for 10 minutes or until meringue is golden.