Adjust the oven rack to the lower third position as the loaf will rise. Preheat the oven to 350°F (177°C).
Grease a 9×5-inch loaf pan with butter or coat with nonstick spray.
In a large bowl, sift the flour, baking soda, salt, and cinnamon together and set aside.
Using a mixer, beat the butter (at room temperature) and brown sugar together on high speed until smooth and creamy, about 2 minutes. On medium speed, beat one egg at a time. Continue to beat until well combined.
Beat in the yogurt or sour cream, mashed bananas, and vanilla extract on medium speed until combined.
Slowly beat the dry ingredients in small batches into the wet ingredients until no flour pockets remain. Do not overmix. Using a spatula, fold in the nuts, if using.
Spoon the batter into the prepared baking pan and bake for 50-60 minutes. Check on the bread after 30 minutes. If the loaf is browning too quickly on top, loosely cover the bread with foil. After 50-60 minutes, insert a toothpick at the center of the loaf. If the loaf is done, the toothpick will come out clean. If not, check again in 5 minutes. Remove from the oven and allow the bread to cool completely.