Instructions:
- Prepare the Base Mixture: In a medium bowl, whisk together the egg yolks and cornstarch until completely smooth and pale yellow.
- Combine Liquids: In the saucepan, gently whisk together the whole milk and sweetened condensed milk over medium heat until just steaming (do not boil).
- Temper the Eggs: Slowly drizzle about 1 cup of the hot milk mixture into the egg/cornstarch mixture while whisking constantly to temper the eggs.
- Cook the Custard: Pour the tempered egg mixture back into the saucepan with the remaining milk. Whisk continuously over medium heat.
- Thicken: Continue whisking until the mixture thickens considerably and begins to bubble vigorously. Cook for exactly one minute while whisking to fully activate the starch.
- Finish Custard: Remove from heat immediately. Whisk in the 2 tablespoons of butter and 2 teaspoons of vanilla until smooth and glossy. Transfer custard to a clean bowl, press plastic wrap directly onto the surface, and chill slightly (about 30 minutes).
- Prepare Layers: Slice the bananas into 1/4-inch thick rounds. If using homemade topping, whip the heavy cream, powdered sugar, and vanilla until medium peaks form.
- Assembly (The Trifle Method): In your serving dish, create the first layer: roughly one-third of the Nilla Wafers, followed by one-third of the sliced bananas, and then one-third of the slightly cooled custard.
- Repeat Layers: Repeat the layering process twice more (Wafers, Bananas, Custard).
- Chill and Set: Cover the entire dish tightly with plastic wrap and refrigerate for at least 4 hours, allowing the cookies to soften.
- Serve: Top generously with the prepared whipped cream just before serving.