Gather all ingredients. Preheat the oven to 350°F (175°C). Grease a 12-cup muffin tin or line with paper liners.

In a medium bowl, whisk together the flour, baking soda, baking powder, salt, and cinnamon.

In a large bowl, mash the bananas with sugar until smooth, then stir in the egg, melted butter, and vanilla.

Fold the dry ingredients into the wet ingredients until just combined, being careful not to overmix.

Spoon the batter into the prepared muffin cups, filling each about two-thirds full.

Bake for 18–22 minutes, or until a toothpick inserted into the center comes out clean. Cool for 5 minutes in the pan before transferring to a wire rack.