Preheat oven to 425°
In a small bowl, peel and mashed the bananas until they are well pureed. Set that aside.
In another bowl, mix all of the dry ingredients together. Set that aside.
In a medium bowl, add the melted butter and brown sugar. Mix well to combine and get all of the brown sugar clumps out.
Add the eggs, one at a time and mixing well between each egg addition.
Add the sourdough starter and mix well.
Add the bananas and mix well.
Add the dry ingredients and carefully mix a few times. Do not overmix at this point. Just mix until all the ingredients are combined. It's ok if there are still a few lumps.
Add the walnuts if you are using them and fold those in once or twice. Do not overmix.
Line a muffin tin with muffin liners and add the batter to the muffin tin evenly. Bake for 20-25 minutes, depending on your oven. A toothpick should be inserted and come out clean. Allow the muffins to cool on a wire rack and enjoy!