Preheat the oven to 350°F (175°C) and line a muffin tin with liners.
In a large bowl, mix mashed bananas with melted butter.
Add sugar, egg, and vanilla extract. Stir until combined.
Add flour, baking soda, and salt. Mix just until incorporated.
Fold in chocolate chips.
Divide the batter evenly into muffin liners, filling about 3/4 full.
Bake for 18–20 minutes or until a toothpick comes out clean.
Let cool slightly before serving.