Preheat oven to 350 degrees F. Line a baking sheet with parchment paper or a silicone mat.
In a large bowl, make the flax egg by combing the ground flaxseed with water. Stir and set aside for 5 minutes to thicken.
Mash the ripe banana into the bowl with the flax egg. Add the coconut oil, maple syrup, and vanilla. Whisk until well combined.
Add the oat flour, baking soda, salt, and cinnamon. Stir with a spatula until just combined. Fold in chocolate chips and walnuts.
Using a cookie scoop (I did a heaping 1.5 Tbsp), scoop dough onto the prepared baking sheet. Slightly flatten with your palms because these don’t spread much. Top cookies with more chocolate chips and walnuts if you want. Bake for 10-12 minutes. Let it cool on the pan for 10 minutes then transfer to a cooling rack to cool completely. Enjoy!