Prepare the Crust: Preheat the oven to 325°F (163°C). Line the bottom of a 9-inch (23cm) springform pan with parchment paper and grease the sides.
Bake the Crust: Combine the vanilla wafer crumbs, melted salted butter, and sugar in a small bowl. Press the mixture into the bottom and up the sides of the pan. Bake for 10 minutes, then set aside to cool. Wrap the outside of the pan securely with aluminum foil for the water bath. Make the Cheesecake Batter: Reduce the oven temperature to 300°F (148°C). Beat the room-temperature cream cheese, sugar, and flour on low speed until smooth, scraping the bowl as needed. Add the sour cream and banana extract, then mix on low speed until combined. Add Banana: Mix the pureed bananas into the cheesecake batter on low speed until incorporated.
Add Eggs: Add the eggs one at a time, mixing slowly after each addition. Scrape the bowl as needed to ensure an even mixture.
Prepare the Water Bath: Pour the cheesecake batter into the cooled crust. Place the springform pan inside a larger pan and add enough warm water to reach about halfway up the sides of the springform pan, keeping the water below the top of the foil.
Bake the Cheesecake: Bake for 1 hour 30 minutes. The cheesecake should be set while the center remains slightly jiggly.
Cool the Cheesecake: Turn off the oven and leave the door closed for 30 minutes. Then crack the oven door and allow the cheesecake to cool for another 30 minutes.
Prepare the Gelatin: While the cheesecake cools, sprinkle the gelatin over the milk in a small bowl and set aside. Whisk the egg yolks in a medium bowl.
Make the Custard: Heat the 1/2 cup heavy cream, sugar, vanilla extract, and banana extract in a saucepan over medium heat until warm. Remove from the heat and slowly pour the hot cream into the egg yolks to temper them.
Thicken the Custard: Return the tempered mixture to the saucepan and cook over medium heat until thick enough to coat the back of a spoon, or until it reaches about 160°F.
Add Gelatin: Stir the gelatin mixture into the hot custard until smooth. Transfer the custard to another bowl and place it over ice. Cool to room temperature, stirring occasionally to prevent lumps.
Whip the Bavarian Cream: Whisk the cold 2/3 cup heavy whipping cream with the powdered sugar on high speed until stiff peaks form.
Finish the Bavarian Cream: Once the custard is at least room temperature, gently fold in the whipped cream. Remove the foil from the cheesecake and spread the Bavarian cream evenly over the top. Refrigerate for 5–6 hours, until completely cool and firm.
Prepare the Whipped Topping: Remove the cheesecake from the springform pan. Whisk the cold heavy whipping cream, powdered sugar, and vanilla extract on high speed until stiff peaks form.
Decorate: Pipe whipped cream swirls around the edge of the cheesecake and finish with fresh sliced bananas.
Store: Keep the cheesecake refrigerated and, for the best result, consume it within 3–4 days.