Preheat oven to 360° and prepare a 9” x 5” loaf pan.
Add lightly mashed bananas, sugar and vanilla to a bowl. Using a stand or handheld mixer, with the paddle attachment, cream the mixture for about a minute.
Add eggs one at a time until fully incorporated.
Add sourdough starter and mix until fully incorporated.
Mix flour, baking soda, and salt together in a separate mixing bowl. Then in alternating batches mix with the banana mixture.
Add milk and oil, mix. Add chopped walnuts, mix just until combined. Try to avoid over-mixing at this stage.
Pour the batter into the prepared pan.
Bake for 60-65 minutes. The bread will become a rich golden color and a toothpick should come out clean. If your loaf is a nice golden brown but not cooked through, cover with foil and return to the oven.
Let the loaf cool in the pan for 20 minutes, then transfer to a wire rack to cool the rest of the way. Enjoy!