Preheat oven to 400°
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. Mix all the streusel ingredients together and sprinkle it on top of the muffins. 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!