Preheat oven to 375 degrees F. Lightly coat a muffin tin with cooking spray, or line with paper liners.
In a large bowl, combine the wheat bran, whole wheat and all-purpose flours, chia seeds, baking powder, cinnamon, baking soda, and salt. Whisk together and set aside.
Peel bananas and, in a separate bowl, mash them until mostly smooth. Add the buttermilk, eggs, brown sugar, oil, and vanilla. Whisk to combine.
Pour wet ingredients into the dry ingredients and fold gently, just until combined. You want the batter to be even, but not over-mixed.
Divide batter evenly among 12 muffin wells, filling each one almost to the top. Bake for 18-20 minutes, until a toothpick inserted into the center comes out clean. Cool and enjoy!