Preheat oven to 350°F (175°C) and grease or line a 9x5 loaf pan.
In a large bowl, mash bananas until mostly smooth.
Add melted butter, brown sugar, granulated sugar, eggs, vanilla extract, and sourdough discard. Mix until combined.
In a separate bowl, whisk together flour, baking soda, baking powder, salt, and cinnamon.
Add dry ingredients to wet ingredients and stir gently until just combined.
Fold in optional mix-ins if using.
Pour batter into prepared loaf pan and smooth the top.
Bake for 55 to 65 minutes until a toothpick inserted comes out with a few moist crumbs.
Cool in the pan for 10 to 15 minutes, then transfer to a wire rack to cool completely before slicing.