01 - Preheat oven to 350°F. Grease and flour a 9x5-inch loaf pan or line with parchment paper.
02 - Whisk together mashed bananas, eggs, melted butter, buttermilk, and vanilla extract in a large bowl until smooth.
03 - Combine flour, granulated sugar, brown sugar, baking soda, salt, and cinnamon in a separate bowl.
04 - Gently fold the dry mixture into the wet ingredients using a spatula, mixing just until combined. Do not overmix.
05 - Fold in the chopped walnuts or pecans until evenly distributed throughout the batter.
06 - Transfer the batter to the prepared loaf pan and smooth the top. Sprinkle extra nuts on top if desired.
07 - Bake for 50 to 60 minutes, until a toothpick inserted in the center comes out clean or with a few moist crumbs.
08 - Cool in the pan for 10 minutes, then turn out onto a wire rack to cool completely before slicing.