Combine mashed bananas, whole milk, heavy cream, and vanilla extract in an empty Ninja Creami pint container. Using an immersion blender or blender, pulse the mixture until smooth and the bananas are mashed.

Add maple syrup to taste, depending on the sweetness of the ice cream you wish to make.

Place the Ninja Creami lid on the pint of Ninja Creami in the freezer, ensure it sits on a level surface, and freeze for 24 hours.

Once frozen, remove the pint from the freezer and remove the lid. Insert the pint into the Ninja Creami bowl, secure the lid, and lock the bowl into place in the machine, per the instructions for your model.

Choose the Ice Cream function, which will operate for approximately 2 minutes. After the cycle is completed, use a spoon to create a hole in the center of the ice cream, reaching the bottom of the pint.

Combine chocolate chunks and walnuts, then add the mixture into the well. Return the bowl to the machine and utilize the Mix In function to incorporate the nuts and chocolate into the ice cream.