Heat the oven to 350°F, then grease and flour a 9 x 5 x 3-inch loaf pan.

Sift together the flour, baking soda, baking powder, and salt, then set the dry ingredients aside.

In a large bowl, cream the room-temperature butter and sugar well until thoroughly mixed and lighter in color.

Add the eggs and beat well until the mixture is light and fluffy, scraping the sides of the bowl as needed.

Stir in the vanilla.

In a separate bowl, mix the mashed banana pulp with the sour cream.

With the mixer on low speed, add the banana-sour cream mixture and the dry ingredients alternately to the butter mixture, adding a little of each at a time until incorporated and scraping the bowl as needed.

Beat only until combined, then use a wooden spoon to give the batter a final stir and make sure there are no dry flour pockets.

Fold in the chopped walnuts, if using, then pour the batter into the prepared loaf pan and smooth the top gently.

Bake for 50 to 60 minutes, starting to check around 50 minutes, until a cake tester inserted off to the side of the loaf comes out clean.

Be careful not to overbake, then let the loaf cool in the pan for a little while before turning it out to finish cooling, or slice it warm if desired.