Heat oven to 350°F. Prepare 4 to 5-cup soufflé dish or casserole with foil band by cutting 3-inch strip of foil to go around top of dish. Lightly grease dish and strip of foil. With greased side toward inside of dish, secure foil band around top of dish, letting it extend 2 inches above edge of dish.
In medium saucepan, combine sugar, cornstarch and salt. Stir in milk. Cook over medium heat until mixture boils and thickens, stirring constantly. Remove from heat; stir in chocolate and margarine until melted. Stir in vanilla. Add egg yolks 1 at a time, beating well after each addition.
In large bowl, beat egg whites with cream of tartar until soft peaks form. Gently fold in chocolate mixture. Pour into greased soufflé dish.*
Bake at 350°F. for 45 to 50 minutes or until knife inserted near center comes out clean. Remove foil band. Immediately serve soufflé with whipped cream or topping.