Preheat oven to 350 degrees F. Line a 9×5 loaf pan with parchment paper.
In a large bowl, make the flax egg by combining the ground flaxseed and water. Stir and set aside for 5 minutes to thicken.
In a medium bowl, add the flour, baking soda, salt, and cinnamon. Whisk until well combined.
In the bowl of the flax egg, add the ripe bananas. Mash until nice and smooth. Add the brown sugar, sourdough discard, vanilla, coconut oil, and almond milk. Whisk until well combined and smooth.
Add the dry ingredients to the bowl with the wet. Stir with a spatula until just combined (careful not to over mix).
Make cinnamon sugar mixture by adding the sugar and cinnamon to a small bowl. Mix to combine.
Pour half of the batter into the prepared loaf pan. Sprinkle half of cinnamon sugar mixture over the batter. Pour the rest of the batter on top and repeat with remaining cinnamon sugar.
Bake for 60-65 minutes, or until a toothpick inserted in the middle comes out clean. Let it cool in the pan for 15 minutes, then remove loaf and place on a wire rack until completely cooled. Slice and enjoy! Best when eaten fresh, but you can store it in an airtight container on the counter for 3 days or in the fridge, the cinnamon sugar topping just won’t be crisp anymore.