Combine first 5 ingredients in top of a double boiler and beat with an electric mixer until smooth. Place over hot water and cover. Cook, stirring occasionally, until thick, then cool. Stir in the vanilla. Place alternate layers of vanilla wafers, bananas and custard in a 1 1/2-quart casserole. Cover and chill, if desired. Serves 6 (Recipe is from Southern Living's The Deep South Cookbook.)