Combine sugar, flour, and cornstarch in a medium mixing bowl. Mix well, and set aside. Pour milk into a large saucepan; cook over medium heat until candy thermometer registers 160 deg F. Gradually stir one-fourth of hot milk into yolks; stir into reserved dry ingredients, and add to remaining hot milk. Cook, stirring constantly, until mixture thickens and coats the spoon. Remove from heat, and stir in butter and vanilla. Let cool to room temperature. Line bottom of a 13 x 9 x 2-inch baking dish with one-third of the vanilla wafers. Arrange half of banana slices over wafers; top with half of cooled custard. Repeat layers, reserving one-third of wafers to crumble and sprinkle over custard. Chill thoroughly. Yield: 20 to 25 servings.