Preheat the oven to 375°F, and lightly grease a 9 x 5-inch loaf pan. Set aside.
In the bowl of a stand mixer, cream together the butter, granulated sugar, and brown sugar on low speed until light and fluffy, about 3 minutes.
½ cup (113 g) unsalted butter, ½ cup (99 g) granulated sugar, ½ cup (107 g) light brown sugar
Add the eggs and vanilla. Continue beating on low speed until the ingredients are well combined.
2 (100 g) large eggs, 2 teaspoons pure vanilla extract
Next, add the bananas. Beat until the bananas are very well mixed and no large chunks remain. The mixture will appear “separated,” but it will come back together as the remaining ingredients are added.
3 bananas
Pour in the sourdough starter, and mix on low speed just until combined. Then add the baking soda, cinnamon, salt, and flour. Mix just until no dry flour streaks remain. Make sure to scrape the sides and bottom of the bowl to ensure all ingredients are incorporated.
¾ cup (175 g) sourdough starter, 1 teaspoon baking soda, 1 teaspoon ground cinnamon, ½ teaspoon salt, 2 cups (240 g) all-purpose flour
Pour the batter into the prepared loaf pan and bake for 15 minutes. Reduce the heat to 350°F, and continue to bake for an additional 40 to 45 minutes or until a knife inserted in the center comes out clean. If the bread begins to brown quickly, make a foil tent and drape loosely over the top.
Remove the pan from the oven, and allow the bread to cool inside the pan for 5 to 10 minutes before transferring to a wire rack. Cool completely before slicing before slicing.