Preheat the oven to 180 degrees C / 375 degrees F.
Add the dry ingredients (gluten-free flour, baking powder, baking soda, cacao powder) to a mixing bowl. Whisk together.
In a separate mixing bowl, whisk together the maple syrup, flax egg, apple cider vinegar, and mashed bananas.
Add the wet ingredients to the dry, and mix together well without over-mixing. Then, fold in the chocolate chunks
Transfer the batter to a rectangular loaf tin (either non-stick, lightly greased, or lined with baking paper). Bake in the preheated oven for around 45 minutes, until hard around the edges but with a good amount of softness on the inside.