Position oven rack in the lower third and preheat to 350°F (175°C). Grease a 9x5 inch loaf pan and line with parchment paper, leaving an overhang on the long sides for easy removal.
Peel the overripe bananas into a large mixing bowl and mash with a fork or potato masher until mostly smooth with a few small lumps — you should have about 1½ cups (360g).
Whisk the melted coconut oil into the mashed bananas, then whisk in the sugar, eggs one at a time, and vanilla extract until smooth and well combined.
In a separate medium bowl, whisk together the flour, baking soda, salt, and cinnamon until evenly distributed with no lumps.
Pour the dry ingredients into the wet ingredients and use a rubber spatula to fold gently until just combined with no dry streaks — do not overmix.
If using, gently fold in the macadamia nuts and coconut flakes, reserving a small handful of each to sprinkle on top.
Pour the batter into the prepared loaf pan, smooth the top with a spatula, sprinkle on reserved toppings, and tap the pan gently on the counter to release air bubbles.
Bake for 55 to 65 minutes until deep golden brown and a toothpick inserted in the center comes out clean or with just a few moist crumbs; tent loosely with foil in the last 15 minutes if the top browns too quickly.
Place the pan on a wire rack and let the bread cool in the pan for 15 to 20 minutes, then use the parchment overhang to lift it out onto the rack.
Allow the bread to cool completely (about 30 to 45 minutes) before slicing with a serrated knife into ¾ to 1-inch thick pieces for clean, even slices.