Preheat your oven to 350°F (175°C) and grease a muffin tin or line it with muffin liners.
In a large mixing bowl, mash the ripe bananas with a fork until smooth.
Mix in the melted butter until fully combined.
Add the sugar, egg, and vanilla extract, whisking until well blended.
In a separate bowl, whisk together the baking soda, salt, ground cinnamon, and flour.
Slowly add the dry mixture to the wet ingredients, stirring just until combined. Do not overmix; a few lumps are okay.
Evenly distribute the batter into the muffin tin.
Bake in the preheated oven for 18-20 minutes, or until a toothpick inserted in the center comes out clean.
Allow the muffins to cool in the tin for 5 minutes before transferring to a wire rack to cool completely.