Preheat oven to 350. Prepare a loaf pan by greasing with vegetable oil spray or butter.
Beat the butter and sugar in a large mixing bowl with an electric mixer on medium speed. Add the egg and vanilla, mixing until combined. Add the mashed banana and sourdough starter, mixing on low.
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.
Pour into loaf pan, smoothing out the top with a spatula, and bake in preheated oven for 1 hour, or until a toothpick comes out clean. The top will split. Allow to cool before slicing. Store at room temperature, tightly wrapped, for up to three days,
To freeze, allow to cool completely and wrap in plastic wrap and then aluminum foil. Keeps in freezer for 2 months.