Heat the oven to 350 degrees F (177 degrees C).
Spray or lightly butter muffin tin.
In a large bowl, whip the sugar and one of the bananas together until well blended.
Add the cooled melted butter, the egg, and the vanilla and beat well.
Mix in the flour, baking soda, and salt just until incorporated.
Peel and mash the remaining banana with a fork on a small plate. Fold into the batter.
Scoop the batter into the muffin tins and fill them ¾ of the way.
Bake until a toothpick stuck in the muffins comes out clean, 20-25 minutes.
