PREHEAT oven to 350°F. Mix 1/2 cup of the sugar, flour and salt in top of double boiler. Blend in 3 egg yolks and milk. Cook, uncovered, over boiling water 10 to 12 minutes or until thickened, stirring constantly. Remove from heat; stir in vanilla.
RESERVE 12 of the wafers for garnish. Spread small amount of custard onto bottom of 1-1/2-quart baking dish; cover with layers of 1/3 each of the remaining wafers and sliced bananas. Pour about 1/3 of the remaining 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 on high speed of electric mixer until soft peaks form. Gradually add remaining 1/4 cup sugar, beating until stiff peaks form. Spoon over custard; spread evenly to cover entire surface of custard and sealing well to edge.
BAKE 15 to 20 minutes or until lightly browned. Cool slightly. Top with reserved 12 wafers just before serving.