1) Mix together the sugar and cornflour and slowly add the 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.
2) Slightly beat the egg yolks and temper with a small amount of the hot custard; stir well. Add the egg mixture to the custard pot and cook for two more minutes. Remove from the heat and add the vanilla and butter. Let cool.
3) In a 23-by-23 cm ovenproof baking dish, layer the pudding, bananas and wafers, beginning with the pudding and ending with the pudding. Add the meringue topping, if desired.
Beat the egg whites and cream of tartar until soft peaks form. Gradually add the sugar until stiff peaks form, then spread on top of the pudding mixture. Bake until golden brown.