Preheat the oven to 350°F (177°C).
Grease the muffin tin with cooking spray or a light coating of butter.
Whip the sugar and one banana in a large bowl until the mixture is well blended.
Add the cooled melted butter, egg, and vanilla, then beat the mixture thoroughly.
Stir in the flour, baking soda, and salt until just combined.
Mash the remaining banana on a small plate and fold it into the batter.
Scoop the batter into the muffin tin, filling each cup three-quarters full.
Bake for 20 to 25 minutes or until a toothpick inserted into the center comes out clean.