In a large saucepan, combine 1/3 cup sugar, cornstarch and salt; stir until blended.
Mix in milk. Cook over medium heat, stirring constantly until thickened and boiling.
Boil for 1 minute, then remove from heat.
In a small bowl, whisk egg yolks, then whisk in 1/2 cup of hot custard until
blended. Pour yolk mixture back into saucepan of custard; cook over medium heat,
stirring, 2 minutes.
Stir in butter and vanilla extract until blended.
Place vanilla wafers on bottom of a shallow 2-quart casserole dish. Top with
layer of banana slices and custard. Repeat layering, ending with custard.
In a large mixing bowl, beat egg whites and 1/4 cup sugar at low speed until
frothy. Add cream of tartar; increase speed to medium and gradually beat in remaining
sugar. Beat until egg whites hold stiff peaks.
Spoon meringue over hot custard immediately, making sure that meringue touches
baking dish on all sides (to prevent it from shrinking).
Transfer to oven and bake
until golden, about 20 minutes.