Preheat the oven to 375°F (190°C) and prepare a muffin pan with liners or grease lightly.

Mash the overripe bananas in a large mixing bowl until smooth.

Stir in the beaten egg, cooled melted butter, and vanilla extract into the mashed bananas until well combined.

In a separate bowl, whisk together the flour, brown sugar, baking soda, and salt.

Gently fold the dry ingredients into the wet banana mixture until just combined.

Fold in optional chocolate chips or walnuts.

Fill each muffin liner about ¾ full with batter.

Bake for 15 to 21 minutes until a toothpick comes out with a few loose crumbs.

Allow muffins to cool in the pan for about 5 minutes before transferring to a wire rack.