Preheat the oven to 350°F (175°C) and grease or line your muffin tin with paper liners.
Peel the ripe bananas and mash them in a bowl until smooth. The riper the bananas, the sweeter and more flavorful your muffins will turn out!
In another bowl, combine the sourdough discard, melted butter, sugar, egg, vanilla extract, milk, and mashed bananas. Combine all the ingredients until the mixture is smooth and fully blended.
In a separate bowl, whisk the flour, baking soda, baking powder, salt, and cinnamon (if using). Slowly incorporate the dry ingredients into the wet mixture, stirring carefully to combine.
Scoop the batter into the muffin tin, filling each cup approximately three-quarters full.
Bake for 22 minutes, or until a toothpick comes out clean and the muffins are golden on top.
Allow the muffins to rest in the tin for 5 minutes. Enjoy!