Preheat oven to 350°F (175°C). Grease a mini loaf pan (8x4) or line muffin cups.
Mash the banana in a bowl until smooth.
Add melted butter, sugar, egg, and vanilla. Mix until combined—don’t overmix.
In another bowl, whisk flour, baking soda, baking powder, salt (and cinnamon if using).
Pour the dry ingredients into the wet. Stir gently until just combined.
Fold in chocolate chips or nuts if using.
Pour into your mini loaf pan.
Bake for 28–35 minutes (or 18–22 min for muffins) —until a toothpick comes out clean.