Preheat oven to 425°.
Pour condensed milk into an 11- x 7- inch baking dish. Cover with foil. Place covered dish in a 13- x 9-inch baking dish; add hot water to dish to a depth of 1 inch.
Bake at 425° for 45 to 55 minutes or until milk is caramel colored and about the consistency of unbeaten egg white (add hot water to 13-x 9-inch dish as needed). Remove 11-x 7-inch dish from hot water bath. Remove foil; set aside. Reduce oven temperature to 350°.
Combine graham cracker crumbs, 1 tablespoon sugar, flour, and 3 tablespoons melted butter; stir well. Divide graham cracker mixture evenly between 9 (6- ounce) custard cups coated with cooking spray; press crumb mixture firmly into bottoms of cups. Place custard cups on a baking sheet; bake at 350° for 8 to 10 minutes or until lightly browned. Set aside; let cool.
Layer banana slices evenly over graham cracker crust in custard cups. Spoon reserved caramel evenly over banana in each cup; set aside.
Combine 1/3 cup sugar, cornstarch, and salt in a medium sauce-pan; stir in milk. Bring to a boil over medium heat, stirring constantly. Cook, stirring constantly, 30 seconds or until thickened. Gradually add a small amount of hot milk mixture to egg yolk, stirring constantly with a whisk. Return egg mixture to pan; cook over medium-low heat, stirring constantly, 1 minute or until thick. Remove from heat; stir in 1 teaspoon butter and vanilla. Pour custard mixture evenly over caramel layer. Cover; chill 3 hours. Top each serving with 1/4 cup whipped topping.