Preheat the oven to 350ºF.
Grease a mini muffin pan and set aside.
Add the bananas to a large bowl and mash them well.
Mix in the melted butter, sugar, egg, and vanilla extract. Stir until well combined.
Mix in the salt, baking soda, and flour. Stir until well combined.
Scoop batter into the mini muffin pan so that each up is about 3/4 full.
Place in the oven and bake for 12-14 minutes.
Remove from the oven and let the muffins cool 4-5 minutes before removing from the pan.