Combine mashed bananas, whole milk, heavy cream, and vanilla extract in Ninja Creami pint (or other small bowl).
Use an immersion blender to pulse a few times, breaking up any chunks of banana (alternatively, you can use a regular blender for this).
Taste the mixture to see if it needs added sweetener. If so, add maple syrup and stir.
Close lid on Ninja Creami pint of liquid and transfer to a level location in your freezer. Freeze for 24 hours.
After freezing, remove pint from freezer and take off lid. Insert pint into Ninja Creami bowl and secure lid. Lock bowl into place in the machine.
Select the Ice Cream function, which will run for about 2 minutes.
After cycle is complete, use a spoon to create a well in the center of the ice cream, about 1.5 inches wide and reaching to the bottom of the pint.
Mix together chocolate chunks and walnuts, then add mixture to the well.
Return bowl to the machine and use the Mix In function to incorporate nuts and chocolat einto the ice cream.