Combine sugar, cornstarch, and salt in top of a double boiler; stir in milk, and bring water to a boil.
Reduce heat to low; cook, stirring constantly, 10 to 12 minutes or until slightly thickened.
Beat egg yolks until thick and lemon colored.
Gradually stir about one-fourth of hot mixture into yolks; add to remaining hot mixture, stirring constantly.
Cook, stirring constantly, until mixture thickens.
Remove from heat, and gently stir in butter and 1 teaspoon vanilla.
Layer half of vanilla wafers in a 2 1/2-quart baking dish; top with half of bananas.
Pour half of filling over bananas.
Repeat layers.
Combine egg whites (at room temperature) and cream of tartar; beat until foamy.
Gradually add 1/4 cup plus 2 tablespoons sugar, 1 tablespoon at a time, beating until stiff peaks form.
Beat in remaining 1/2 teaspoon vanilla.
Spread meringue over filling, sealing to edge of dish.
Bake at 350° for 10 to 12 minutes or until golden brown.
Let cool to room temperature; chill.