A tender, ultra-moist banana bread enriched with brown sugar and butter, warmly spiced with cinnamon, and bursting with ripe banana flavor—equally stunning plain or studded with toasted nuts.
Adjust oven rack to the lower third position and preheat to 350°F (177°C). Grease a metal 9×5-inch loaf pan with nonstick spray.
Whisk together the flour, baking soda, salt, and cinnamon in a medium bowl; set aside.
With a handheld or stand mixer, cream the butter and brown sugar on high speed until smooth and creamy, about 3 minutes.
With mixer on medium speed, beat in the eggs one at a time, mixing well after each addition. Beat in the yogurt (or sour cream), mashed bananas, and vanilla until combined.
With mixer on low speed, slowly mix the dry ingredients into the wet ingredients just until no flour pockets remain; do not overmix. Fold in nuts if using.
Pour batter into the prepared pan. Bake 60–65 minutes, tenting loosely with aluminum foil halfway through to prevent over-browning. Bread is done when a toothpick inserted in the center comes out clean with just a few moist crumbs; begin checking around 60 minutes.
Cool in the pan on a wire rack for 1 hour, then remove from the pan and cool completely on the rack. Slice and serve.
Storage: Cover and store at room temperature up to 2 days or refrigerate up to 1 week. Bread tastes best on day 2. For freezing, wrap well and freeze up to 3–4 months.