Preheat oven to 350°F (175°C). Line a muffin tin with paper liners or grease lightly.
In a large bowl, mash the ripe bananas until smooth. Stir in the melted butter.
Mix in sugar, beaten egg, and vanilla extract until well combined.
Add baking soda and salt, then gently fold in the flour. Do not overmix.
Stir in optional add-ins like chocolate chips or nuts.
Divide the batter evenly into 12 muffin cups (about 3/4 full).
Bake for 18–20 minutes, or until a toothpick inserted in the center comes out clean.
Allow muffins to cool for 5 minutes in the tin, then transfer to a wire rack.