Preheat your oven to 350°F (175°C). Grease a muffin tin or line with muffin liners.
In a large bowl, mix the mashed bananas, melted butter, and sugar until well combined.
Add the beaten eggs and vanilla extract to the banana mixture and mix well.
In another bowl, whisk together the flour, baking soda, and salt.
Gradually add the dry ingredients to the wet ingredients, mixing until just combined.
Pour the batter into the prepared muffin tin, filling each cup about ¾ full.
Bake for 20-25 minutes, or until a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.