Mix together sugar and cornstarch and slowly add milk. This should be cooked in the top of a double boiler, but you can cook it over low to medium heat, stirring constantly until it thickens--do not leave it unattended. Slightly beat egg yolks and temper with a small amount of the hot custard; stir well. Add egg mixture to custard pot and cook 2 more minutes. Remove from heat and add vanilla and butter. Let cool. In a 9 by 9-inch oven proof baking dish, alternate pudding, bananas, and wafers, beginning with pudding and ending with pudding. Add topping, if desired.
For the meringue topping, if using:
Beat egg whites and cream of tartar until soft peaks form. Gradually add sugar until stiff peaks form, then spread onto top of pudding mixture. Bake until golden brown.