Preheat oven to 350F (175C) and grease and flour the bottom of a 9×5 bread loaf pan (or line with a sling of parchment paper).

In a large mixing bowl, combine mashed bananas, sourdough discard, sugar, melted butter, eggs, and vanilla extract and stir very well.

In a separate mixing bowl, whisk together flour, baking powder, baking soda, salt, and cinnamon (if using).

Add about half of the flour mixture to the banana mixture and gently fold together with a spatula. Once just combined, add milk, and stir until the milk is combined, then add the remaining flour mixture and stir until just combined. Do not over-mix or bread will be dense and rubbery.

Pour batter into prepared loaf pan and sprinkle the surface with sugar (if desired).

Transfer to center rack of oven and bake for 60-65 minutes or until a wooden skewer inserted into the center comes out clean or with moist crumbs.

Allow bread to cool in pan for 15-20 minutes, then use a knife to gently loosen the sides from the pan and carefully turn out onto a cooling rack to cool completely.