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