Tip: Adding just a tiny bit of water to the pan helps prevent the top of the cake from cracking as it bakes.
Preheat the oven to 350 degrees.
Fill a pan that is large enough to fit a 9-inch springform pan with ½-inch water and place it on the center rack in the oven.
Line the inside of a 9-inch springform pan with parchment paper. Wrap a sheet of aluminum foil around the outside of the pan so that water cannot get into the pan when it is submerged in the water bath in the oven.
Add the Nilla wafers to the bowl of a food processor and crush into a fine crumb. The cookies should be the consistency of sand with no chunks. Mix the melted butter with the cookie crumbs and press the crumbs into the bottom of the springform pan and up the side, 1-1 ½ inches. Place the pan in the freezer while you make the cheesecake filling.
Combine the softened cream cheese with the sugar using an electric mixer on high speed. When the mixture is smooth, add the eggs, sour cream, vanilla extract, and banana flavoring and beat well.
Use a spatula to pour all of the filling into the springform pan. Smooth the top, then gently place the pan into the water bath in the oven and bake until the cheesecake filling does not jiggle in the middle when moved, about 70 minutes.
Let cool, then cover and chill in the refrigerator while you make the Bavarian cream.
Heat the milk and cream in a medium-sized saucepan over medium heat until boiling. Remove from the heat. Combine the egg yolks with the sugar in a medium-sized bowl and beat with an electric mixer on high speed until the mixture is pale yellow, about 1 minute. Add the hot liquid a little bit at a time to the eggs while beating.
Pour the mixture back into the pan and heat over low heat, stirring often until the mixture thickens just enough to coat the back of a spoon, about 8 minutes. Be sure that the mixture does not boil or the eggs will curdle.
Pour back into the medium-sized bowl. Dissolve the gelatin in cold water in a bowl, let it sit for 1 minute, then mix it into the Bavarian cream with an electric mixer on high speed along with the vanilla extract and banana flavoring.
Chill the Bavarian cream for 1 hour, then spread it over the top of the cheesecake. Cover and chill the cheesecake.
Combine all of the ingredients except for the bananas in a medium-sized bowl. Beat with an electric mixer on high speed until the cream forms stiff peaks. Cover and chill in the refrigerator.
When the cheesecake has completely chilled out, about 4 hours, cut it into 12 slices and serve each with a dollop of the banana whipped cream on top, with 1 slice of banana on top of the whipped cream, and ½ slice of banana on the side of each serving.