Prep oven and pans. Preheat oven to 425°F (218°C). Spray a 12-count muffin pan with nonstick spray or use cupcake liners.
Combine dry ingredients. Whisk the flour, baking powder, baking soda, salt, cinnamon, together in a medium bowl. Set aside.
Combine wet ingredients. Mash the bananas. Whisk in the melted butter, brown sugar, egg, vanilla extract and sourdough discard.
Combine wet and dry ingredients. Pour the dry ingredients into the wet ingredients, then whisk until combined. Do not overmix. I stop mixing just as the last of the dry ingredients are stirred in. If adding nuts or chocolate chips, stir them in now. Batter will be thick.
Bake muffins. Spoon the batter into liners or greased muffin tin, filling them all the way to the top. Bake for 5 minutes at 425°F then, keeping the muffins in the oven, reduce the oven temperature to 350°F. Bake for an additional 16-18 minutes or until a toothpick inserted in the center comes out clean or with a few moist crumbs. Allow the muffins to cool for 5 minutes in the muffin pan, then transfer to a cooling rack to continue cooling.