Cream together the butter, sugar, egg, and vanilla.
Mix in the sourdough starter and mashed bananas.
In a separate bowl combine the flour, salt and baking soda. Add the flour mixture to banana mixture and mix just until combined. Do not over-mix.
Pour the batter into a parchment paper lined 9x5-inch bread pan. Bake at 350°F for 1-hour and 10-minutes or until a toothpick inserted into the center of the loaf comes out clean. Cool completely prior to slicing.
Edited by Lonetree