Preheat oven to 350°F (175°C). Line a muffin tin with paper liners or grease it.

In a large bowl, mash the bananas until mostly smooth. Stir in brown sugar, melted butter (or oil), egg, and vanilla until combined.

In another bowl, whisk together flour, baking soda, baking powder, salt, and cinnamon (if using).

Gently fold dry ingredients into wet mixture until just combined—don’t overmix.

If using, stir in milk or yogurt for extra moisture.

Spoon batter evenly into the muffin tin (about ⅔ full).

Bake for 18–22 minutes, or until a toothpick inserted in the center comes out clean or with a few moist crumbs.

Let muffins cool in the tin 5 minutes, then transfer to a wire rack to cool completely.