2In 1 1/2 quart casserole dish, mix together sugar, cornstarch, cocoa, salt and 1/2 cup of milk until smooth. Once it is mixed well, stir in remaining milk. Microwave on high for 5 to 8 minutes or until thick; stir halfway through cooking.
3In a small bowl, place a small amount of the hot mixture and beat together with egg yolks. Stir back into original mixture in casserole dish. Microwave on high for 1 1/2 to 2 minutes or until thickened; stir frequently. Blend in butter and the 1 teaspoon of vanilla. Pour mixture into pastry shell.
4To Make Meringue: In a small bowl, beat egg whites and cream of tartar until foamy. Gradually stir in 2 tablespoons of sugar at a time until stiff peaks form; stir in vanilla. Spread meringue over top of pie filling.
5Seal rim of pie with aluminum foil, then bake in preheated oven for about 8 minutes or just until meringue gets browned.