Mix 1/2 cup sugar, flour, salt in top of double boiler. Blend in 3 egg yolks and milk. Cook uncovered over boiling water, stirring constantly for 10-12 minutes or until thickened. Remove from heat; stir in vanilla.
Reserve 10 wafers for garnish. Spread small amount of custard on bottom of a 1 1/2 quart casserole. Cover with a layer of wafers and a layer of sliced bananas. Pour about 1/3 of custard over bananas. Continue to layer wafers, bananas and custard to make a total of 3 layers each, ending with custard.
Beat egg whites until soft peaks form and gradually add remaining 1/4 cup of sugar and beat until stiff but not dry. Spoon on top of pudding, spreading evenly to cover the entire surface and sealing well to edges.
Bake at 350 in top half of the oven for 15 to 20 minutes or until browned. Cool slightly or refrigerate. Garnish with additional wafers and banana slices before serving.