Add the sweetened condensed milk and the water to a large bowl. Mix until combined with a hand mixer.
Add the vanilla pudding mix and the vanilla extract to the bowl. Mix until well combined, it will be runny.
Cover the bowl with plastic wrap and refrigerate for 2 hours or until the pudding is fully set.
When the pudding is ready, pour the heavy cream into a large mixing bowl. Whip the cream until stiff peaks form, just be careful not to overmix it.
Add spoonfuls of the pudding mixture to the whipped cream with the mixer on low speed until the pudding is fully incorporated with no steaks. I like to take a rubber spatula and scrape the bottom of the bowl to make sure it's all combined.
Transfer the pudding mixture to a serving bowl, if desired. Stir in the crushed Nilla wafers, mini marshmallows, and sliced bananas. Top the salad with more cookies, marshmallows, and bananas for garnish, and serve immediately.
Notes
If you want to use Cool Whip in place of the whipped cream, you'll need 16 ounces. However, the pudding will be extra sweet since Cool Whip has sugar in it already.
You can use regular-sized wafers if that is all you can find, just crush them up a bit more.
This fluff is really best served right away, or as soon as possible. The cookies will start to absorb the pudding mixture and become very thick. So, leftovers will have a different texture, and not be as fluffy, but let me tell you it is still out of this world delicious, so don’t throw it away! Just keep in mind if serving for a party. Also, the marshmallows will slightly dissolve and the bananas will begin to brown in the coming days.
Leftovers can be kept in the fridge for up to 5 days.