Melt butter in a saucepan over medium heat, or in the microwave, until just melted. Set aside to cool for a few minutes.
Preheat your oven to 350°F (175°C). Prepare a 9x5 loaf pan by greasing and lining with a parchment sling (optional).
Whisk together whole wheat flour, oats, baking powder, baking soda, cinnamon and salt in a large bowl.
Mash bananas in a separate bowl until smooth (I like to leave some chunks for texture) and then mix in the sourdough starter, maple syrup, eggs, and vanilla. Stir until well combined.
Using a Danish whisk or a wooden spoon, mix wet ingredients into dry ingredients until just combined.
Finally, pour in the melted butter and mix until just incorporated.
Fermentation: You can choose to bake your sourdough banana bread immediately, or you can let it ferment prior to baking. To allow your sourdough banana bread to ferment simply leave the mixing bowl, covered with plastic wrap, on the counter for anywhere from 4-24 hours. Keep in mind that the longer the batter ferments, the easier your bread will be to digest. If you prefer, you can place the batter in the fridge for 24 hours up to 3 days and bake at your convenience!
Fill your prepared loaf pan with the batter. It should fill the pan about 3/4 of the way up.
Bake for 35-45 minutes or until a knife inserted comes out clean. I like to gently touch the center top of the loaf with a finger, it should feel solid and set, and no longer jiggly.
Allow the sourdough banana bread to cool for 5-10 minutes in the pan, then use a knife to go around the edges of the bread before tipping the loaf out of the pan. Cool until it reaches room temperature on a wire rack.
Slice and enjoy with butter!