Mix the dry ingredients. In a medium bowl, whisk together the flour, baking soda, salt, and cinnamon. Set aside.
Cream butter and sugar. In a large bowl with a stand or hand mixer, beat the butter and brown sugar on medium-high for 2–3 minutes, until pale and fluffy.
Add the eggs. Add the eggs one at a time, mixing well after each addition. Scrape down the bowl.
Add the wet ingredients. Mix in the sour cream, mashed bananas, and vanilla until just combined. The batter may look slightly curdled — that's fine.
Combine. Add the dry ingredients to the wet and fold gently with a spatula just until no streaks of flour remain. Do not overmix.
Fold in nuts. Stir in the chopped pecans or walnuts.
Fill the pan. Grease a mini loaf pan with non-stick spray. Fill each cavity about two-thirds full (don't overfill — the bread will rise).
Air fry. Place the pan in the air fryer basket. Set the temperature to 320°F and air fry for 22–28 minutes, until the top is golden brown and a toothpick inserted in the center comes out with a few moist crumbs.
Cool. Let the bread rest in the pan for 10 minutes, then turn out onto a wire rack to cool completely before slicing.