Make the flax eggs. In a small bowl, whisk together the ground flax and water until combined. Allow to sit for at least 10 minutes.
In a separate large bowl or stand-up mixer, combine the hazelnut flour or almond flour, coconut flour, tapioca starch or arrowroot starch, coconut sugar, baking powder, baking soda, and sea salt. Stir to blend.
In the small bowl with the flax eggs, add the almond butter or tahini, almond milk, coconut nectar or maple syrup, and vanilla extract. Whisk until combined.
Incorporate the almond butter mixture into the large bowl with the flour mixture. Start to beat on medium speed. Add the melted coconut oil, increasing the speed as you need until a sticky batter is formed. Refrigerate the dough for 15-20 minutes.
Preheat the oven to 350 degrees. Line two baking sheets with parchment paper.
Once the dough has chilled, use a cookie scoop or your hands to form cookies of desired size. Bake for 14 to 15 minutes, turning pan halfway through to ensure even baking. Cookies will have started to turn golden brown and still be soft to the touch, but they will firm as they cool.