Melt butter in a microwave safe dish, in a saucepan, or small cast iron pan.
Lightly grease a muffin tin or line with cupcake liners.
In a stand mixer with a paddle attachment or a bowl, mix together melted butter and brown sugar for five minutes.
Turn off the mixer and add in eggs, vanilla, mashed banana, and sourdough starter.
In a separate bowl, stir together dry ingredients (flour, baking soda, baking powder, and salt) well.
Add in dry ingredients 1/3 at a time and mix until just incorporated. You don’t want to over-mix.
Add equal amounts of batter to each muffin well. I like to use an ice cream scoop for this.
Top each muffin with the oat crumble.
Bake for 20-25 minutes or until a toothpick or cake tester comes out clean.
Allow the muffins to cool completely. Enjoy
Notes
* If you want to long ferment this recipe, add an extra 1/2 cup of sourdough starter to. Otherwise the batter won't be moist enough for the ferment. Long fermented directions in the post above.
For a more tangy flavor, use sourdough discard that hasn’t been fed in a day, or allow to long ferment. For a less tangy flavor, use an active sourdough starter.
Crumble topping is optional, but adds a super yummy flavor and texture.
I highly suggest serving them warm. They taste best, in my opinion, right out of the oven.