In a large bowl, mash the ripe bananas with a fork until mostly smooth with a few small lumps remaining. Add the melted butter, egg, sour cream, and vanilla extract, then whisk everything together until well combined.
In a separate bowl, whisk together the flour, sugar, baking powder, baking soda, salt, and cinnamon. Pour the dry ingredients into the banana mixture and stir gently with a spatula until just combined.Fold in the chocolate chips if using. Be careful not to overmix, as this will keep the muffins tender.
Line silicone muffin cups or small ramekins with paper liners if desired. Fill each cup about two-thirds full with batter. This recipe makes about 8 muffins depending on the size of your molds.
Preheat the air fryer to 325 degrees F. Place the muffin cups in the air fryer basket in a single layer, working in batches if needed to avoid overcrowding.
Cook for 13 to 15 minutes until the tops are golden and a toothpick inserted in the center comes out clean. The lower temperature ensures they bake evenly without browning too quickly on the outside.
Let the muffins cool in their cups for 5 minutes before removing them to a wire rack. Serve warm or at room temperature. They are incredibly moist thanks to the sour cream and bananas.