Preheat the oven to 350 degrees. Line a muffin pan with cupcake liners.
Melt butter.
In a stand mixer; mix melted butter and brown sugar for five minutes.
Turn off the mixer; add in mashed banana, eggs, vanilla, and sourdough discard.
In a separate bowl; stir together flour, baking soda, baking powder, and salt.
Add in dry ingredients ⅓ at a time and mix until just incorporated - you don’t want to over-mix!
Add batter to each muffin liner.
Make oat crumble and top each muffin; mix together brown sugar, oats, flour, softened butter, cinnamon.
Bake for 20-25 minutes or until a toothpick or cake tester comes out clean. Allow the muffins to cool completely.
