Mix milk and ground flax in a small bowl and set aside for a few minutes to gel.
In a large mixing bowl, whisk melted vegan butter and cashew butter.Tip:If your cashew butter is hardened from being in the fridge, microwave for 10-15 seconds until runny.
Add granulated sugar and whisk until well combined.
Add flax mixture and vanilla extract and whisk to combine.
Add oat flour, potato starch, baking soda, and cream of tartar. Mix until a thick, cohesive dough forms.
Cover the dough and chill in the fridge for at least 30 minutes and up to 24 hours.
Preheat oven to 350°F/180°C. Line a cookie sheet with parchment paper or a silicone baking mat*.
Mix together cinnamon and sugar in a small bowl.
Scoop about 1½ tbsp of dough and roll into balls, then roll in cinnamon sugar. Place on baking sheet with at least two inches of space between each dough ball.
Bake for 9 minutes. Drop the cookie sheet a few inches from the top of the baking rack once to deflate the cookies, then remove from the oven.
Let the cookies cool on the sheet for 2 minutes, then transfer to a wire rack to cool completely.