Preheat the oven to 350 degrees F. Grease/spray your donut pan very well.
Make flax egg by combining the ground flaxseed and water in a small bowl. Let it sit for 5 minutes.
In a small bowl, make cinnamon sugar topping by adding coconut sugar and cinnamon and stir. Set aside.
In a medium bowl, combine the oat flour, cinnamon, salt, and baking powder. Whisk until well combined.
In a smaller mixing bowl, add the flax egg, mashed banana, oil, milk, maple syrup, and vanilla extract. Whisk until well combined.
Add the wet ingredients to the dry and gently stir with a spatula until just combined. Don’t overmix. Spoon batter into your donut pan. Mine made about 5-6 donuts. Sprinkle cinnamon sugar topping over the top.
Bake for 15-18 minutes or until a toothpick comes out clean. Let it cool for 5 minutes in the pan and then carefully take the donuts out and place on a wire rack to cool. Enjoy! Best when fresh!