Combine. Beat the sweetened condensed milk and ice water on medium speed until thoroughly combined. Add the vanilla pudding mix and beat for another 5 minutes until the pudding has totally dissolved.
Chill. Cover the pudding mixture and chill in the fridge for at least 3-4 hours. You can also let it chill overnight and finish it in the morning.
Make the whipped cream. In a large bowl, beat the heavy cream on medium-high speed until stiff peaks form.
Put it all together. Gently fold the pudding into the whipped cream until thoroughly combined (no streaks left behind).
Assemble. Spread 1/3 of the pudding into the bottom of a 4-5 quart trifle bowl. Arrange a layer of Nilla wafers (laid flat) over the pudding. Follow the wafers with a single layer of sliced bananas. Sprinkle with ½ teaspoon cinnamon. Repeat and then spread a final layer of pudding over the top. Sprinkle with the remaining cinnamon.
Chill. Cover the pudding and chill in the fridge for 4 hours.
Serve. Pull the pudding out of the fridge and uncover it. Add a couple of Nilla wafers and extra slices of banana to the top, if desired. Serve chilled.
Notes
If you don't have any vanilla wafers on hand, you can also use graham crackers in their place.