Mix ½ cup sugar, flour and salt in top of double boiler. Blend in 3 egg yolks and milk. Cook, uncovered, over boiling water, stirring constantly for 10 to 12 minutes or until thickened. Remove from heat; stir in vanilla.
Reserve 10 wafers for garnish. Spread small amount of custard on bottom of 1 ½-quart casserole; cover with a layer of wafers and a layer of sliced bananas. Pour about ⅓ of custard over bananas. Continue to layer wafers, bananas and custard to make a total of 3 layers of each, ending with custard.
Beat egg whites until soft peaks form; gradually add remaining ¼ cup sugar and beat until stiff but not dry. Spoon on top of pudding, spreading evenly to cover entire surface and sealing well to edges.
Bake at 350°F in top half of oven for 15 to 20 minutes or until browned. Cool slightly or refrigerate. Garnish with additional wafers and banana slices just before serving.