In a large mixing bowl, whisk together the sweetened condensed milk and the ice-cold water until smooth. Add the instant vanilla pudding mix and whisk for about 2 minutes until fully combined and no lumps remain.
Cover the bowl tightly with plastic wrap and place it in the refrigerator for at least 4 hours (or overnight) until the pudding base is very firm. Do not skip this step!
In a separate large bowl or stand mixer, whip the cold heavy cream on medium-high speed until stiff peaks form.
Take your firm pudding mixture out of the fridge. Using a rubber spatula, gently fold the pudding mixture into the whipped cream until completely smooth and no yellow streaks remain.
In a 9x13 baking dish or large trifle bowl, spread a small amount of the pudding mixture on the bottom. Layer 1/3 of the Nilla Wafers evenly over the pudding.
Layer 1/3 of the sliced bananas over the wafers. Top with 1/3 of the pudding mixture, spreading it out evenly.
Repeat these layers two more times (Wafers, Bananas, Pudding).
Cover the dish tightly with plastic wrap and refrigerate for 4 to 8 hours before serving. Top with crushed Nilla Wafers right before serving.
