Mix 1/2 cup sugar, flour and salt in top of double boiler. Add 3 egg yolks and milk. Cook uncovered, over boiling water 10-12 minutes, stirring constantly. Remove from heat; stir in vanilla.
Reserve 12 of the vanilla wafers for garnish. Spread small amount of custard on bottom of baking dish; cover with layers of 1/3 each of the remaining wafers and sliced bananas and 1/3 of the remaining custard over bananas. Continue to layer wafers, bananas and custard to make a total of 3 layers each, ending with custard.
Beat egg whites on high speed until soft peaks form. Gradually, add remaining 1/4 cup sugar, beating until stiff peaks form. Spread evenly over custard to cover; sealing well to edge. Bake 15-20 minutes until browned.
Cool slightly. Insert the 12 remaining wafers along inside edge of cooked pudding. Serve.