Preheat the oven to 180 °C (160 °C fan). Line or grease a 12-cup muffin tin.
In a mixing bowl, whisk together mashed bananas, sugar, cinnamon, egg, melted butter, and vanilla extract until smooth.
In another bowl, combine flour, baking soda, and salt. Fold the dry ingredients into the banana mixture until just combined—don’t overmix.
Divide batter evenly among the muffin cups.
Bake for 20–25 minutes, or until muffin tops are golden and a toothpick inserted in the center comes out clean.
Cool in the tin for 5 minutes before transferring to a wire rack to cool further.