Grease a 9×5 inch loaf pan and set it aside.

Peel the bananas and mash them in a large mixing bowl with a fork or potato masher until mostly smooth with small lumps.

Add 1 cup sourdough starter, granulated sugar, brown sugar, eggs, melted butter or oil, and vanilla to the mashed bananas and whisk until the mixture looks smooth and well combined.

In a medium mixing bowl, whisk together flour, baking soda, baking powder, salt, and cinnamon if using until everything is evenly distributed.

Pour the dry mixture into the bowl with the wet ingredients and stir gently with a rubber spatula just until no dry flour streaks remain; the batter should be thick but pourable.

Fold in nuts or chocolate chips if using, stirring only a few times so you do not overmix.

Scrape the batter into the prepared loaf pan with the rubber spatula and smooth the top so it spreads evenly from edge to edge.

Place the pan on the middle rack of a preheated 350°F oven using oven mitts and bake for 50 to 60 minutes until the top is deep golden, a crack runs down the center, and a toothpick or thin knife inserted into the middle comes out clean or with just a few moist crumbs.

Remove the loaf pan from the oven with oven mitts and let the banana bread cool in the pan for 10 minutes.

Run a knife around the edges if needed, then turn the loaf out onto a rack or cutting board and let it cool at least 30 minutes before slicing so the crumb sets and stays moist, not gummy.