Preheat the oven to 350°F (176°C) and prepare a 9" x 5" loaf pan by lightly greasing it. In a mixing bowl, combine the flour, sea salt, baking powder, and baking soda, whisking them together thoroughly. In a separate container, mash the ripe bananas until smooth using a fork and set aside. Melt the butter in a microwave-safe bowl. Into the melted butter, incorporate the brown sugar, eggs, sourdough starter, vanilla extract, sour cream, and the mashed bananas. Stir these components together with a spatula until the batter becomes homogenous. Gradually fold the dry ingredients into the wet mixture, stirring just enough to combine without overmixing. If desired, gently integrate chopped nuts. Transfer the batter into the prepared loaf pan, smoothing the top. Bake in the center of the oven for 50 to 60 minutes, or until a toothpick inserted into the center emerges clean. Once baked, let the loaf cool in the pan for 15 minutes. Run a knife carefully along the edges to loosen it, then lift it onto a wire rack. Allow the bread to cool completely before slicing.