Preheat oven to 350° F. Prepare a 9x5 bread loaf pan with nonstick spray or parchment paper.
In a large bowl, gently whisk flour, baking soda, baking powder, and salt. Add chocolate chips and walnuts to flour mixture. Toss to evenly coat.
In a separate medium bowl, mash banana, then add buttermilk, brown sugar, oil, eggs, and vanilla and whisk to combine.
Add buttermilk mixture to flour mixture. Using a spatula, fold ingredients just until moistened. Do not overmix.
Pour batter into the prepared bread loaf pan. Bake 50-55 minutes or until slightly golden and a toothpick inserted into the middle comes out clean. Enjoy!