Preheat the air fryer to 320°F (160°C). Lightly grease a small loaf pan that fits in your air fryer basket.
In a large bowl, mash the bananas until smooth. Add melted butter, sugar, egg, and vanilla extract, then whisk until well-combined.
In a separate bowl, whisk together flour, baking powder, and salt.
Gently fold the dry ingredients into the wet mixture until just combined. Do not overmix.
Pour the batter into the prepared pan, spreading evenly.
Place the pan in the air fryer basket and cook for 30 minutes, or until a toothpick inserted comes out clean.
Let cool for 10 minutes on a wire rack, then remove from pan. Slice and enjoy!