In a large mixing bowl, combine the instant vanilla pudding mix and whole milk. Whisk together until the mixture is smooth and begins to thicken, about 2 minutes.
Add the Nutella to the pudding mixture. Using a spatula or whisk, blend until the Nutella is fully incorporated and the pudding is a rich, chocolatey color.
In a separate bowl, whip the heavy cream until stiff peaks form. Gently fold the whipped cream into the Nutella pudding mixture until well combined.
In your prepared baking dish, start layering the dessert. Begin with a layer of crushed Oreo cookies at the bottom, followed by a layer of the Nutella pudding mixture, and then a layer of sliced bananas. Repeat the layers until all ingredients are used, finishing with a layer of pudding on top.
Spread the Cool Whip evenly over the top layer of pudding. This will create a fluffy, creamy finish that complements the richness of the Nutella.
Cover the dish with plastic wrap and refrigerate for at least 4 hours, or overnight if possible. Just before serving, sprinkle the chopped roasted hazelnuts on top for an added crunch and nutty flavor.
