01 - Whisk whole milk, granulated sugar, cornstarch, and salt in a medium saucepan until smooth.
02 - Place saucepan over medium heat, stirring constantly until the mixture thickens, approximately 5 minutes.
03 - Lightly beat egg yolks in a separate bowl. Gradually whisk in 1/2 cup of the hot milk mixture to slowly raise the temperature.
04 - Slowly pour the tempered egg mixture back into the saucepan, whisking continuously. Cook for an additional 2 to 3 minutes until the pudding is thick and creamy.
05 - Remove saucepan from heat. Stir in unsalted butter and vanilla extract until the pudding is fully smooth.
06 - Let pudding cool slightly, covering directly with plastic wrap to prevent a skin from forming.
07 - In a 2-quart trifle dish or glass bowl, create layers starting with vanilla wafers, followed by banana slices and warm pudding. Repeat the layers, finishing with a layer of pudding.
08 - Cover assembled pudding and refrigerate for at least 2 hours, or until thoroughly chilled.
09 - In a mixing bowl, whip heavy cream, powdered sugar, and vanilla extract with an electric mixer until soft peaks form.
10 - Just before serving, spread or pipe whipped cream over the pudding. Garnish with extra banana slices or crushed wafers as desired. Serve chilled.