In small mixing bowl, put sugar, cornstarch, salt, cocoa, and flour, mix well. Slowly add some of boiling water to dry sugar mixture. Mix well until smooth. Add this mixture to rest of boiling water. Return to stove and cook until thick. Remove from stove. Pour a small amount of this mixture over well beaten egg yolks. Add egg mixture to balance mixture in pan and cook about 1 to 2 minutes. Remove from heat and add butter and vanilla. Mix well. Pour into prepared baked pie shell. Meringue: Beat egg whites until foamy, add salt and cream of tartar. Continue beating until fairly stiff. Add sugar, one tablespoon at a time. Arrange meringue on pie and bake at 350 degrees for 15 minutes, or until nicely browned.