Preheat oven to 350 degrees F. Line a 8x8 square pan with parchment paper.
In a large bowl, make the flax egg by combining the ground flaxseed with water. Set aside for 5 minutes to thicken.
In a medium bowl, add the flour, cacao powder, baking soda, baking powder, and salt. Whisk until well combined.
In the bowl with the flax egg, add the ripe bananas and mash until smooth. Then add the maple syrup, coconut sugar, milk, apple cider vinegar, vanilla extract, and hazelnut butter. Whisk until well combined and smooth.
Add the dry ingredients to the wet. Stir with a spatula until just combined. Careful not to over mix. Fold in the chocolate chips, saving a few for the top.
Pour batter into the prepared pan. Sprinkle chopped hazelnuts all over the top. Sprinkle some chocolate chips on top too. Bake for 35 minutes or until a toothpick inserted in the middle comes out mostly clean. Let it cool completely before slicing. Enjoy!