Preheat oven to 350 degrees F. Grease a 9x5-inch loaf pan with nonstick cooking spray and set aside.
To brown the butter, put butter in a small saucepan and melt over medium heat. Once melted, turn up the heat to medium high. Continue stirring and look for small golden bits that will start to settle on the bottom of the pan. It will smell deliciously nutty and caramel-y. This should take around 3 to 5 minutes. Once this happens, take it off the heat and pour into a medium, heat-safe bowl to cool to room temperature.
In a medium bowl, whisk together flour, baking powder, baking soda, cinnamon, and salt. Set aside.
In a large bowl, combine the mashed bananas with the dark brown sugar and mix until smooth. Stir in the eggs until well combined. Pour in the buttermilk, browned butter, and vanilla extract and mix until combined.
Add the dry ingredients and mix until just combined, don’t over mix. Gently fold in 2/3 cup of the chopped nuts.
Pour the batter into the prepared loaf pan and sprinkle turbinado sugar and remaining chopped nuts over the top. Bake until golden brown and a toothpick or cake tester comes out clean, about 50 to 65 minutes. Check early to be safe, but mine always takes 60 to 65 minutes to cook through. If the top gets too dark, you can loosely cover the loaf with foil and finish baking.
Remove the bread from the oven and let sit in pan for 10 minutes. Loosen with a bread knife around the edges and carefully remove the bread from the pan. Cool completely on a cooling rack before slicing.