Slice the bananas into rounds and place in a medium bowl. Lightly toss with lemon juice to prevent browning.
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 with a whisk.
Remove from heat; whisk in 1/2 tsp vanilla and butter.
Spread small amount of custard onto bottom of 1-1/2 to 2-qt. baking dish.
Reserve 10-12 nilla wafers to garnish.
Cover with a layer of 1/3 each of the remaining wafers, then bananas, and remaining custard. Repeat layers 2 times.
Heat oven to 350°F.
Beat egg whites on high using an electric mixer until soft peaks form.
Add 1/4 cup sugar and 1/2 tsp vanilla, beating until stiff but not dry.
Spoon over custard; spread evenly, sealing well to edge of dish.
Bake at 350°F for 15 to 20 minutes or until lightly browned.
Cool slightly (or refrigerate several hours) until chilled.
Top with reserved wafers just before serving.