Preheat oven to 350F.
Mix together the ground flaxseed with 2 tablespoons of water to create a flax egg. Set aside for a few minutes to thicken.
Cream the room temperature butter in the bowl of a stand mixer with the whisk attachment. You can also do this by hand but it'll take a bit more time and energy.
Add the light brown sugar to the creamed butter and continue to beat until well combined. Then, add the flax egg and 1 tablespoon of the plant-based milk.
Whisk together the properly measured all purpose flour, baking soda, salt, cinnamon and cream of tartar in a separate bowl. Add the dry mixture to the butter mixture and beat on low until combined. If the dough is crumbly or dry, add the other tablespoon of plant milk.
Stir together the cinnamon sugar coating. Roll the cookie dough in to two tablespoon sized balls and drop the balls in to the cinnamon sugar and toss to coat. Make sure they are completely covered. Place on parchment lined baking tray leaving room in between each cookie.
Bake the snickerdoodle cookies for 10 minutes. Bang the cookie tray on the counter a few times to help flatten. Sprinkle with remaining cinnamon sugar and let cool for a few minutes before enjoying!