Preheat the oven to 350°F. Line an 8½ × 4½-inch loaf pan with parchment paper and grease lightly with coconut oil. (You can also use a 9 × 5-inch loaf pan; the finished loaf of banana bread will be a little thinner.)
In a large mixing bowl, combine the mashed bananas, coconut sugar, almond milk, chocolate hazelnut butter, coconut oil, and ground flax seed. Whisk well to combine. The batter should be smooth with only a few small chunks of banana.
In a separate bowl, whisk together the hazelnut flour, oat flour, cacao powder, baking soda, baking powder, and salt. Using a rubber spatula, fold the dry ingredients into the wet ingredients and stir until well mixed. Fold in the chocolate chips.
Pour the batter into the prepared loaf pan. Top with hazelnuts or chocolate chips, or both, if using.
Bake for 50 minutes to 1 hour, until a toothpick or knife inserted near the middle of the loaf comes out clean. If you’re using a 9 × 5-inch pan, check on the bread after 40 to 45 minutes.
Let the bread cool completely in the pan before removing and slicing. Store the bread, tightly wrapped in plastic wrap or foil or in a sealed container on the counter, for up to 3 days. Refrigerate for up to 1 week or freeze for up to 6 months. I like to freeze slices that I can heat in the microwave whenever I want a slice of this bread!