Preheat your oven to 350°F (175°C).
In a mixing bowl, combine the melted butter and mashed bananas.
Stir in brown sugar, egg, and vanilla extract until well combined.
In a separate bowl, whisk together the flour, baking soda, and salt.
Gradually fold the dry ingredients into the wet mixture.
Gently stir in the chocolate chips.
Spoon the batter into a lined muffin tin, filling each cup about 2/3 full.
Bake for 18-20 minutes, or until a toothpick comes out clean.
Let the muffins cool in the tin for 5 minutes, then transfer to a wire rack.
