In a saucepan over medium heat, melt the unsalted butter. Continue cooking, stirring frequently, until the butter turns a golden brown color, about 5-7 minutes. Let it cool slightly.
In a large mixing bowl, combine the dark brown sugar and granulated sugar. Pour in the warm brown butter and mix until well combined. Add the large eggs one at a time, mixing well after each addition. Stir in the vanilla extract.
Fold in the sour cream and mashed overripe bananas until just combined. Be careful not to over-mix.
In a separate bowl, whisk together the all-purpose flour, baking powder, baking soda, kosher salt, ground cinnamon, ground nutmeg, and grated orange zest (if using).
Gradually add the dry ingredients to the wet mixture, folding gently with a rubber spatula until just combined. If using walnuts, fold them in at this stage.
Preheat your oven to 350°F (175°C). Grease your loaf pan or line it with parchment paper. Pour the batter into the prepared pan, smoothing the top with a spatula.
Bake in the preheated oven for 60-70 minutes, or until a toothpick inserted into the center comes out clean. Keep an eye on it towards the end of the baking time.
Once baked, let it cool in the pan for about 10 minutes. Then, transfer it to a cooling rack to cool completely. Slice and enjoy!
