Preheat the oven to 375 degrees F and line two baking sheets with parchment paper or lightly spray with oil.
Make the flax egg: Combine ground flaxseed and water in a small bowl and set aside to thicken.
Make the topping: In a small bowl, combine the sugar and cinnamon and stir. Set aside.
In a large bowl, cream together the vegan butter and sugar for about 1-2 minutes using a handheld mixer, stand mixer, or even just a wooden spoon. Mix in the flax egg and vanilla, scraping the sides as needed.
To the bowl with the wet ingredients, add the flour, cream of tartar, baking soda, cinnamon and salt. Turn on the mixer (or mix by hand) and mix until just combined. Do not over mix.
The dough will be quite thick and not too wet or dry. You should be able to roll the dough in balls. If you added too much flour and the dough is crumbly, add a tablespoon or two of non-dairy milk. If too wet for some reason, add a tablespoon of flour. This is a good way to troubleshoot any cookie recipe.
Take about 2 tablespoons of dough and roll into a ball. Repeat, and roll the balls in the cinnamon-sugar topping. Place on baking sheets and bake for 11 minutes. The cookies will be very soft. Let them cool for about 10 minutes, then transfer to a cooling rack.