Preheat oven to 350 degrees F. Lightly grease a 9’’ or 10’’ pie plate with cooking spray.
Add Nilla wafers to a food processor and pulse until crumbs. (If you don’t have a food processor you can crush them in a ziplock bag, with a rolling pin.)
Add cookie crumbs to a bowl and stir in melted butter.
Press mixture firmly into the pie plate and up the sides.
Bake for 10 minutes.
Allow to cool completely on a wire rack before adding the custard filling.
For the filling:
Whisk eggs and yolk together in a bowl. Set aside.
Add sugar, salt and cornstarch to a large, heavy saucepan and whisk to combine. Add cold milk and stir well.
Turn heat on to medium-high and cook, whisking constantly (so that the sugar and milk don’t burn), for 5 minutes.
Continue stirring until mixtures comes to a gentle boil. Maintain this gentle boil over medium heat, cooking for 5-7 minutes, until thickened.
Add a large spoonful of the hot mixture into the bowl with the eggs, and stir constantly as you add it. This is called “tempering” the eggs. You add the hot mixture a little at a time to the eggs to slowly warm them, without “scrambling” them.
Repeat, with several more spoonfuls of hot custard, stirred into the eggs.
Add the tempered egg mixture to the saucepan, stirring constantly, until incorporated.
Cook, stirring, over medium heat for 2 more minutes.
Remove from heat and stir in butter and vanilla extract.
Pour custard into a bowl and lay a piece of plastic wrap onto the top of the custard to prevent a thick film from forming on it as it cools.
Refrigerate for 1 hour.
After the custard has cooled, spoon half of it into the cooled pie crust. Add a layer of thinly sliced bananas*. Add remaining custard and smooth into an even layer.
Add heavy cream to a mixing bowl and whip until peaks form, then beat in powdered sugar.
Smooth whipped cream over the top of the pie,
Refrigerate, uncovered, for at least 4-6 hours before serving.
Storing Instructions: Cover the pie in plastic wrap and store it in the refrigerator for up to 3 days. Make Ahead Instructions: Prepare the Nilla wafer crust several days in advance. The finished pie can be stored in the fridge for 2-3 days before serving. If you do plan to make the pie in advance, I would suggest NOT adding bananas to the middle of the pie. Instead, add all of the custard to the pie, and refrigerate it. When ready to serve, serve each pie slice with a dollop of whipped cream and fresh bananas slices. This way you don't have to worry about the bananas getting mushy or brown.Freezing Instructions: Allow the pie to cool complete. Wrap it in plastic wrap and a layer of tinfoil and freeze for up to 2 months. Thaw overnight in the refrigerator. Serve cold.