
Note: I like to let the muffin batter rest for 30 minutes before baking it, this helps give us nice tall muffin tops. If you want to skip the rest, preheat your oven to 425F (220C) here.
In a large mixing bowl, whisk together mashed bananas, sourdough discard, butter, sugars, egg, and vanilla extract.
In a separate, medium-sized mixing bowl, whisk together flour, baking soda, baking powder, and salt.
Add the dry ingredients to the wet and use a spatula to gently fold together until just combined. Be careful not to over-mix or muffins could be dense and dry!
Cover batter with a dry towel and let rest for 30 minutes. While the batter rests, preheat your oven to 425F (220C) and line a 12-count muffin tin with paper liners.
Once 30 minutes has passed, divide batter evenly into muffin tin. Sprinkle tops with coarse sugar.
Bake muffins in preheated oven for 8 minutes, then without opening the oven door reduce oven temperature to 350F (175C) and continue baking for another 7-8 minutes, until a toothpick inserted into the center of a muffin comes out dry or with a few moist crumbs.
Allow muffins to cool in muffin tin for 5-10 minutes, then remove to a cooling rack.