Preheat the oven: Preheat the oven to 350°F. Line a light colored aluminum 9x5-inch loaf pan with parchment paper or grease well.
Brown the butter: For a detailed tutorial, see this post on how to brown butter. Pour the butter in a large mixing bowl and allow the butter to cool slightly, ~10-15 minutes. Start Timer 1/2 cup unsalted butter
Mix the butter and sugar: Whisk together the browned butter, granulated sugar, and oil until well combined, about 30 seconds.
1 cup granulated sugar, 1 Tbsp. vegetable oil
Add the remaining wet ingredients: Then whisk in the eggs, mashed bananas, buttermilk, and vanilla until smooth.
2 large eggs, 1¼ cups mashed overripe bananas, 2 tsp. vanilla extract, 1/3 cup buttermilk
Combine dry ingredients: In a separate bowl, whisk together the flour, baking soda, baking powder, salt, cinnamon, and nutmeg.
2 cups all-purpose flour, 1 tsp. baking soda, 1/2 tsp. baking powder, 1 tsp. kosher salt, 1/2 tsp. ground cinnamon, 1/4 tsp. ground nutmeg
Finish the batter: Add the dry ingredients to the wet ingredients and gently fold until just combined. Do not over-mix. Let this rest for 5-10 minutes to hydrate the flour, fold again 2 times after it rests. Start Timer
Bake: Pour the batter into the prepared loaf pan. Bake for ~60 minutes, covering the top with foil about halfway through to prevent over-browning. The banana bread is done when a toothpick inserted into the center comes out clean with a few moist crumbs. Start Timer
Cool: Let the bread cool in the pan for 10 minutes, then transfer to a wire rack to cool completely before slicing. YUM! This bread stays really moist and tender even on days 2-3! Start Timer