Combine 1/2 cup sugar, flour and salt in top of double boiler. Stir in 4 egg yolks and milk; blend well. Cook, uncovered, over boiling water, stirring constantly until thickened. Reduce heat and cook, stirring occasionally, for 5 minutes. Remove from heat; add vanilla. Spread small amount on bottom of 1 1/2 quart casserole; cover with layer of Nilla Wafers. Top with layer of sliced bananas.
Pour about 1/3 of the custard over bananas. Continue to layer wafers, bananas and custard to make 3 layers each, ending with custard.
Beat egg whites until stiff but not dry; gradually add remaining 1/4 cup sugar and beat until stiff peaks form. Spoon on top of pudding, spreading to cover entire surface and sealing well to edges. Bake at 425 degrees for 5 minutes or until delicately browned. Cool slightly or chill.
Just before serving, garnish with banana slices, then stand Nilla Wafers upright around edge of dish. Makes 8 servings.