Preheat oven to 350°F (180°C). Line a 12-cup muffin pan with paper liners.
In a medium bowl, combine flour, baking soda, baking powder, and salt. Set aside.
In another bowl, mash the bananas, then add the melted butter and sugars. Whisk until combined.
Add eggs, vanilla extract, and buttermilk.
Add the dry ingredients to the wet ingredients and mix until just combined.
Spoon batter into prepared muffin cups.
Bake for 20–25 minutes, until a toothpick, inserted into the center, comes out with only a few moist crumbs attached.
Let cool for 10 min in the pan, then transfer to a cooling rack to cool completely.