Grease a 7-inch (18 cm) round cake pan or line it with parchment paper to prevent sticking. You can also use a small loaf pan that would fit in your air fryer but you might need to half the recipe or cook in 2 batches.
In a medium bowl, mash the ripe bananas until smooth. A few lumps are fine, as they add texture to the bread.
In a separate bowl, combine all-purpose flour, baking powder, and salt. If using self-raising flour, skip the baking powder.
Whisk the room temperature eggs with sugar using a stand mixer or a hand mixer until the mixture is pale yellow for about 5-6 minutes.
Add the mashed bananas to the egg mixture and whisk until combined, and then add the oil and the milk. Whisk the mixture to combine for about 30 seconds.
Gently fold in the dry ingredients along with the chocolate chips and chopped walnuts, and then pour it into the prepared pan.
Place the loaf pan in the air fryer basket and bake at 320°F (160°C) for 25-30 minutes. Reduce the heat to 300℉ (150℃) and bake for another 10 minutes, or until a toothpick inserted in the middle of the loaf comes out clean.
Once done, let the bread cool in the pan for about 10 minutes before transferring it to a wire rack. Allowing it to cool fully helps it set and makes slicing easier.