Preheat the oven to 350 degrees F. Using a food processor, crush the Nilla wafers into fine crumbs. (You can also crush them in a gallon food storage bag with a rolling pin.) Pour the crumbs into a medium mixing bowl. Pour in the melted butter and stir until the crumbs are evenly moistened.
Press the mixture into an 8-inch pie dish. Bake for 5 minutes. Then, remove from the oven and allow to cool on a wire rack.
Filling
Combine sugar, cornstarch, and salt in a 3-quart saucepan. Add the milk and vanilla bean (if using) and stir to combine. Cook over medium heat, stirring constantly until the mixture is bubbling. Continue to cook while stirring for another 10 minutes until the mixture has slightly thickened.
Temper the eggs by adding a small amount of the hot mixture from the saucepan to the egg yolks while whisking constantly. Repeat while whisking until you have a warm, smooth mixture. Add the tempered egg mixture back into the saucepan and cook for a couple of minutes while constantly stirring.
Remove the pan from the heat. Remove the vanilla bean - being sure to scrape out as many of the seeds as possible into the pan - and discard the pod. Stir in the butter and vanilla until fully combined.
Arrange the sliced bananas in the bottom of the cooled pie crust until the bananas are level with the top of the crust. Pour the warm pudding over the bananas. Tilt the pie dish as necessary to help the pudding completely coat the bananas and fill in the gaps. (There might be excess pudding. Just fill as much as possible without spilling.)
Cover the pie with plastic wrap placed directly onto the pudding to prevent a skin from forming. Refrigerate for at least 2 hours so the pudding can cool and set up.
Topping
Add the heavy cream and powdered sugar to a medium mixing bowl. Beat with a hand mixer until medium peaks form.
Remove the pie from the refrigerator and remove the plastic wrap. Top with whipped cream. Add additional banana slices on top of the pie if desired. Refrigerate until ready to serve.
Notes
This pie is best enjoyed the same day it's made. The bananas will brown and get mushy as time passes.