In a large bowl, mix the flour with the coconut flour, baking powder, and sea salt.
Make chia eggs. To a small bowl, add the chia seeds and the water, then let rest for about 7-8min, until the mix is viscous.
In another bowl, mash the bananas. Melt the butter and mix it with the bananas, then add sugar and cinnamon.
Add the chia eggs and give it a stir until just combined.
Add the rest of the ingredients to the banana mix. To do so, alternate the dry ingredients, half a cup at a time, and the milk, 1/4 cup at a time. Stir until well combined.
Bake in a pre-heated oven at 370F for 50min.