Prep. Preheat oven to 350. Prepare a loaf pan by greasing with vegetable oil spray or butter.
Beat butter and sugar. Beat the butter and sugar in a large mixing bowl with an electric mixer on medium speed.
Add egg and vanilla. Add the egg and vanilla, mixing until combined. Scrape down the bowl with a spatula and mix again.
Add bananas and starter. Add the mashed banana and sourdough starter, mixing on low.
Dry ingredients. In a separate medium mixing bowl mix the flour, salt, baking soda, and baking powder. Add it to the wet ingredients and mix on low until just combined. Overmixing will make the loaf tough.
Bake. Pour into a loaf pan, smoothing out the top with a spatula, and bake in a preheated oven for 60-75 minutes, or until a toothpick comes out clean. The top will split.
Cool and slice. Allow to cool before slicing. Store at room temperature, tightly wrapped, for up to three days.