Preheat your oven to 350°F (175°C). Grease or line a muffin tin with paper liners.
In a large mixing bowl, mash the ripe bananas with a fork until smooth.
Stir the melted butter into the mashed bananas until well combined.
Mix in the baking soda and salt.
Stir in the sugar, beaten egg, and vanilla extract until fully incorporated.
Add the baking powder and flour, and mix gently until just combined. Be careful not to overmix; some lumps are fine.
Divide the batter evenly among the 12 muffin cups.
Bake for about 20 minutes, or until the muffins are golden brown and a toothpick inserted into the center comes out clean.
Let the muffins cool in the tin for about 5 minutes, then transfer them to a cooling rack to cool completely.