Preheat oven to 375°F (190°C). Grease or line a muffin tin.
In a bowl, combine bran cereal and milk. Let sit for 5 minutes.
In another bowl, mix mashed bananas, oil, egg, brown sugar, and vanilla.
Add the bran mixture to the banana mixture.
In a separate bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon.
Add dry ingredients to wet ingredients, mixing until just combined.
Spoon batter into muffin tin, filling each cup about ⅔ full.
Bake for 18-20 minutes or until a toothpick inserted into the center comes out clean.
Let cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.