Preheat oven to 350°F and line a 9x9 baking pan with parchment paper, leaving an overhang to allow for easy removal.
Crush vanilla wafers with a food processor, or place in a gallon freezer bag and crush with a rolling pin. Pour into a medium bowl.
Add granulated sugar and stir to combine.
Add melted butter and stir well until mixture is well saturated with butter.
Pour into baking pan and press evenly into the bottom of the pan.
Bake in preheated oven for 10 minutes. Remove from oven and let cool.
Reduce oven temperature to 325°F.
In a large bowl, purée overripe bananas with a hand mixer or immersion blender until smooth.
Beat in granulated sugar and cream cheese, mixing well until smooth.
Add eggs, vanilla, and heavy cream and beat until smooth after each addition.
Add cornstarch and mix well. Pour into cooled vanilla wafer crust and smooth on top.
Bake in preheated oven for 35 minutes, or until outside of pan is puffed but center of cheesecake still jiggles slightly.
Cool completely at room temperature, then cover and chill at least 4 hours or overnight.
Custard and Toppings
Heat evaporated milk and butter in a medium saucepan over medium heat, stirring until butter is melted and mixture reaches a simmer. There should be small bubbles near the outside of the pan, but don't let it boil. Turn off heat.
In a medium bowl, whisk together egg yolks, sugar, and cornstarch until smooth. Add about half of scalded milk mixture very gradually, ½ cup at a time, pouring slowly and whisking the egg mixture constantly. This is to temper the eggs and prevent them from curdling.
Return the saucepan to medium heat and gradually pour the tempered egg-and-milk mixture back into the pan, whisking constantly.
Cook custard mixture, stirring constantly with a whisk or wooden spoon, for 10-15 minutes, until mixture thickens and coats the back of a spoon. Remove from heat and set aside.
Top cheesecake layer with fresh banana slices and mini vanilla wafers. Pour slightly cooled custard on top, and cut into squares. Alternatively, you can cut the cheesecake squares first and top each with custard individually.
Sprinkle with more vanilla wafer crumbs, if desired. Serve immediately or refrigerate until ready to serve, keeping in mind these squares are best served the same day.
Make ahead: You can make the cheesecake ahead of time and freeze it until you're ready to make the custard -- just store in the fridge for up to 3 days or in the freezer, well wrapped, for up to 2 months.