Preheat the oven to 350°F (175°C).

Line a muffin tin with parchment paper or muffin liners.

In a mixing bowl, mash the ripe bananas with a fork or potato masher until smooth.

Add the melted butter to the mashed bananas and mix until combined.

In a separate bowl, mix together the baking soda, salt, and all-purpose flour.

Stir the sugar, beaten egg, and vanilla extract into the banana mixture.

Gently fold the dry ingredients into the wet mixture until just combined; avoid overmixing.

Pour the batter evenly into the prepared muffin tin, filling each cup about two-thirds full.

Bake in the preheated oven for 18 to 20 minutes, or until a toothpick inserted into the center comes out clean.

Remove the muffins from the oven and let them cool in the tin for 5 minutes, then transfer to a wire rack to cool completely.