Preheat oven to 350. Soften butter by either leaving it out for an hour or so, or 10-30 seconds on a low microwave setting, it's best when soft but not melted. Combine the flour, salt, and baking soda by hand. With mixer on a medium to high speed, beat the softened butter and sugar until pale yellow and creamy. Mix in eggs. Add flour mixture and beat until smooth. Peel the bananas and break into chunks, add to the mixer bowl along with the sour cream or yogurt. Beat using medium speed until smooth Add walnuts and mix on low until evenly distributed in the batter. If making bread, prepare the bread pan by spraying with Pam or buttering the pan. Scrape the batter into the pan, distribute the batter evenly in the pan, and bake at 350 for 50-60 minutes. Test for doneness by inserting a knife blade in the center, if the blade comes out with uncooked batter sticking to it, bake for another 5-10 minutes. If making muffins, prepare the muffin pan(s) by either using paper muffins cups, or spraying with Pam. Distribute the batter evenly, usually makes 12-14 regular size muffins. Bake at 350 for 25 minutes, then test one of the muffins in the middle of the tray for doneness as described above. If not done yet, bake for another 5 minutes.