Remove the peel from the bananas and add them to a high-speed blender, along with the peanut butter, milk, and vanilla extract. Secure the lid and blend until very smooth; there should be no visible chunks.
Pour the blended mixture into a Ninja creami pint container. The mixture should not go higher than the "max fill" line. Secure the lid and place the pint container on a flat shelf in your freezer. Let it freeze for 24 hours or more.
Once the mixture is frozen solid, you can spin it in your Ninja Creami whenever you're ready. (You can freeze this for at least 1 month, so feel free to prep it as early as you'd like to.)
Remove the lid from the pint container and place it into the outer container with the handle. Click the lid, with the blender blade attached, into place. Then secure the outer bowl in the Ninja Creami machine. (You should hear a click when it is correctly placed.) Press the Sorbet button to start the first cycle.
When the cycle is complete, the banana ice cream most likely won't look creamy yet. Secure it back into the machine and run a re-spin cycle. This should make it much creamier! (You'll know it's good when there's a hole in the center of the ice cream.) If it's still not as creamy as you'd like, you can run a re-spin cycle a second time.
Serve this creamy banana ice cream right away with any toppings you love. Turn it into a sundae with homemade magic shell and crunchy peanuts on top.
Notes
Nutrition information is for roughly 2/3 cup of ice cream, assuming you make 2 cups total. This information is automatically calculated using generic ingredients, so it's just an estimate, not a guarantee. Milk Note: I used soy milk when testing this recipe, but almond milk, coconut milk, regular milk, or oat milk will also work. (Even water will work; the bananas and nut butter add plenty of creaminess.)Nut Butter Note: Peanut butter and banana is a popular flavor combination, but you can use any other nut or seed butter you love. If you want to leave it out, the ice cream will be less creamy.