Preheat the oven to 350°F (175°C). Place the Nilla wafers in a food processor and pulse until fine crumbs form. Transfer the crumbs to a bowl and mix with the melted butter until evenly combined. Press the mixture firmly into the bottom and up the sides of a 9-inch pie plate. Bake for 10 minutes. Remove from the oven and allow to cool completely.

In a medium bowl, whisk together the egg yolks and cornstarch until smooth. Set aside. In a large saucepan, whisk together the milk, sugar, and salt. Cook over medium heat, stirring frequently, until the mixture begins to simmer.

Slowly whisk a ladleful of the hot milk mixture into the egg yolk mixture, whisking constantly. Gradually pour the tempered egg mixture back into the saucepan, whisking continuously.

Continue cooking over medium heat, stirring constantly, until the custard thickens enough to coat the back of a spoon. Remove from the heat and stir in the butter and vanilla extract. Strain the custard through a fine mesh sieve into a bowl. Cover the surface directly with plastic wrap and allow to cool for 30 minutes.

Arrange the sliced bananas evenly over the cooled crust. Pour the custard over the bananas and smooth the surface. Place plastic wrap directly on top of the custard to prevent a skin from forming. Refrigerate for at least 4–6 hours, or until fully set.

In a mixing bowl, whip the heavy cream and powdered sugar until soft peaks form. Spread evenly over the chilled pie.

Refrigerate until ready to serve. Slice and serve chilled.