Preheat your oven to 350°F (175°C).
Grease the muffin tin with butter or line with paper liners.
Mash the ripe bananas in a large mixing bowl until smooth.
Add softened butter and brown sugar to the mashed bananas, and cream together until light and fluffy.
Beat in the eggs, one at a time, mixing well after each addition. Stir in the vanilla extract.
In a separate bowl, whisk together the all-purpose flour, baking soda, baking powder, and kosher salt.
Gradually add the dry ingredients to the banana mixture, stirring gently with a rubber spatula.
Scoop the batter into the prepared muffin tin, filling each cup about 2/3 full.
Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for 5 minutes before transferring them to a cooling rack.