Preheat the Oven: Preheat your oven to 350°F (175°C) and line a muffin tin with paper liners or grease the cups.
Mix Wet Ingredients: In a large bowl, mix the mashed bananas with melted butter. Stir in the sugar, beaten egg, and vanilla extract until well combined.
Combine Dry Ingredients: In another bowl, whisk together the flour, baking soda, and salt.
Combine Mixtures: Gradually add the dry ingredients to the banana mixture, stirring gently until just combined. Be careful not to overmix; a few lumps are okay.
Add Optional Ingredients: If desired, fold in nuts or chocolate chips.
Fill Muffin Cups: Scoop the batter into the prepared muffin tin, filling each cup about 2/3 full.
Bake: Bake for 18-20 minutes, or until a toothpick inserted into the center comes out clean.
Cool: Allow the muffins to cool in the tin for a few minutes before transferring them to a wire rack to cool completely.