Preheat oven to 325 degrees. Roast walnut pieces for 12 minutes. Remove and allow to cool while preparing batter. Increase oven temperature to 350 degrees. Lightly grease and flour a 9 x 5-inch loaf pan.
In a medium mixing bowl, whisk together flour, cocoa, baking powder, soda, and salt. Set aside.
Using a fork, mash enough bananas to equal 1 1/2 cups, around 3. Dice remaining banana into small chunks. Set aside.
In a large mixing bowl, whisk eggs until slightly whipped. Add coconut sugar and whisk to combine with eggs. Add melted coconut oil, buttermilk, mashed and diced banana, and vanilla and whisk to incorporate. Fold in the flour mixture and stir just until moistened. Add nuts and chocolate chips and stir to combine. Pour batter into loaf pan and bake 55 minutes or until a fork inserted in the center comes out clean. If the fork hits a chocolate chip try again.
Allow to cool for 15 minutes before inverting on a wire rack. Cool completely before flipping over and adding hot fudge sauce.
Hot Fudge Sauce
Combine ingredients in a small saucepan and heat on low until chips are completely melted, about 5 minutes. Stir chips while melting.
Remove from heat and allow to cool slightly before drizzling over the bread. There will be extra hot fudge which can be added to individual slices, if desired.
Cover and refrigerate leftover hot fudge. Fudge will become very thick and may need reheating before eating a second time.