Preheat your oven to 375°F (190°C) and line a muffin pan with paper liners.

In a large mixing bowl, mash the overripe bananas into a smooth puree using a potato masher.

Add the beaten egg, cooled melted butter, and vanilla extract into the mashed bananas and mix until homogeneous.

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

Gradually add the dry ingredients to the wet banana mixture, stirring gently until just incorporated.

Fold in optional mix-ins like chocolate chips or walnuts into the batter.

Spoon the batter into the muffin liners, filling each about ¾ full.

Bake in the preheated oven for 15-21 minutes, or until a toothpick comes out clean.

Remove from the oven, cool in the pan for 5 minutes, then transfer to a wire rack to cool completely.