Preheat oven to 350°F.
Prepare muffin tins with liners.
In the bowl of an electric mixer, cream together the butter, brown sugar and granulated sugar until smooth, about 1-2 minutes.
Add the eggs and mix until combined.
Add the vanilla, mashed bananas, sourdough discard and milk and mix until combined.
In a separate bowl, combine the flour, baking soda, baking powder, salt and cinnamon. Add the flour mixture to the wet mixture and combine until all ingredients are fully incorporated (you may need to stop and scrape the mixer bowl half way through to make sure there are no lumps of unmixed ingredients). Avoid over mixing as this will make the muffins too dense. If you are adding chopped nuts, stir them into the mixed batter.
Scoop the muffin batter into each prepared muffin liner, filling about 2/3 full.
Bake muffins for 22-25 minutes or until a toothpick inserted comes out clean with only a few moist crumbs.
Let muffins cool in the pan for 10 minutes before removing to cool completely on a wire cooling rack.
Enjoy!