For the crust, crush the Graham crackers into medium coarse crumbs using a zipper bag, add melted butter, and mix well. Spray a muffin pan with nonstick cooking spray, divide the crumbs evenly into 12 cups, and press firmly to form the crust. Place in the freezer for 20 minutes.
To prepare the custard, combine sugar, cornstarch, salt, milk, and egg yolks in a medium saucepan. Cook over medium heat, stirring continuously until the mixture thickens and coats a spoon, about 5-7 minutes. Remove from heat, stir in vanilla extract.
Remove the crust from the freezer and evenly distribute the custard among the crusts. Cover loosely with plastic wrap and refrigerate for at least 5 hours.
For the topping, beat the heavy cream with a hand mixer or stand mixer on medium-high until thick. Add powdered sugar and vanilla extract, and continue to beat until soft peaks form. Be careful not to overbeat.
Slice the bananas and place evenly onto each custard-filled pie. Top each pie with whipped cream and serve immediately.