Preheat the oven to 350°F (175°C) and line a 12-cup muffin pan with paper liners.
Mash the ripe bananas in a large bowl.
Add the melted butter, brown sugar, egg, and vanilla extract. Stir until combined.
In a separate bowl, whisk together the flour, baking soda, cinnamon, and salt.
Add the dry ingredients to the banana mixture and gently stir until just combined.
Fold in chocolate chips or chopped nuts if desired.
Divide the batter evenly between the muffin cups, filling each about two-thirds to three-quarters full.
Bake for 18–22 minutes, or until a toothpick inserted into the center comes out clean or with a few moist crumbs.
Cool in the pan for 5 minutes, then transfer to a wire rack.
Serve warm or allow the muffins to cool completely before storing.