Preheat the oven to 425 degrees F and line a muffin tin with muffin liners.
In a large mixing bowl whisk all of the dry ingredients. In a small bowl, mash the bananas well with a fork.
1 and 1/2 cups all purpose flour, 1 tsp baking soda, 1 tsp baking powder, 1/2 tsp salt, 1 tsp ground cinnamon, 1/4 tsp ground nutmeg
In a separate mixing bowl or a stand mixer, beat the melted butter, mashed bananas, sugar, milk, egg, and vanilla. Add in the sourdough discard and beat until combined.
1 and 1/2 cups mashed ripe bananas, 1/3 cup sourdough discard, 1 egg (large), 2 tsp vanilla, 6 tbsp melted butter, 2/3 cup brown sugar (light or dark)
Add the dry ingredients to the wet, and beat until just combined. Use a spatula to scrape down the sides of the bowl.
Fill each cupcake liner to the top with muffin batter.
Bake at 425 degrees F for five minutes. Without opening the oven, reduce the temperature to 350 degrees F and bake for an additional 16-18 minutes or until a toothpick comes out clean.