Start heating water in double boiler. Peel and slice bananas, placing in a lemon juice mixture to refrain from turning brown. Meanwhile in a medium mixing bowl place flour, 1 cup of sugar, salt. Mix well and make a "well" in middle of bowl. Gradually start adding milk and stirring constantly until well mixed. Transfer mixture to the top of double boiler. Cook on med to med-high heat, stirring as needed. Mixture will thicken in approximately 10-14 minutes. When starts to thicken - remove from heat, add vanilla and stir well. Set aside.