The night before, usually before I go to bed, start by mashing the bananas in a bowl, then mix in the flour, sourdough starter, vanilla, salt, butter, oats, and sugar, stir well.
Since your flour and oats are sitting overnight with the sourdough starter, they will be easier to digest. I recommend not letting these muffins ferment for more than 10 hours or the rise when baking won't be as good, and your muffins may turn out a bit flat.
Cover and let your mixture sit overnight.
The next morning, preheat the oven to 350F. Add the remaining ingredients: eggs, baking soda, and baking powder, and mix well. Then, stir in the chocolate chips.
Grease your muffin tin and add batter. Sprinkle sugar and cinnamon on top of each muffin before baking.
Bake for 20 to 25 minutes or until the top is lightly browned. Let cool for 5 minutes in the muffin tin, then move to a cooking rack.