Prep the pan and air fryer: Lightly grease a mini loaf pan and line it with a parchment sling if you have one. Preheat the air fryer to 320°F (160°C) for 3–5 minutes.
Mash the bananas: In a medium bowl, mash the bananas until mostly smooth with a few small lumps. You want a thick puree.
Whisk the wet ingredients: Add melted butter, brown sugar, granulated sugar, egg, and vanilla. Whisk until glossy and well combined.
Combine the dry ingredients: In a separate small bowl, whisk flour, baking soda, baking powder, salt, and cinnamon.
Bring the batter together: Add dry ingredients to the banana mixture. Stir gently with a spatula until just combined. If the batter is very thick, fold in 1–2 tablespoons milk to loosen slightly. Do not overmix.
Fold in extras: If using nuts or chocolate chips, fold them in now. The batter should be thick but spoonable.
Fill the pan: Scrape the batter into the prepared mini loaf pan and smooth the top. For a nice dome, heap slightly in the center.
Air fry: Place the pan in the air fryer basket. Cook at 320°F (160°C) for 25–30 minutes, or until the top is deep golden and a toothpick inserted in the center comes out clean or with a few moist crumbs.
Tent if needed: If the top browns too quickly by the 18–20 minute mark, loosely cover the loaf with a small piece of foil and continue baking.
Cool: Let the loaf cool in the pan for 10 minutes, then lift it out and cool on a rack for at least another 10–15 minutes before slicing. This helps set the crumb.