Preheat oven to 350°F/175°C. Line parchment paper on two cookie sheets.
Vegan Buttermilk: Make the vegan buttermilk by combining plant-based milk with apple cider vinegar. Stir and set aside.
Vegan Egg: Prepare the flax egg by combining ground flax seeds with water. Stir and set aside.
Dry Ingredients: Combine in a large mixing bowl the flour, cocoa, salt, baking soda, and cornstarch. Stir to combine.
Whipped Butter: In a mixing bowl beat the butter until light and fluffy. Add the sugars and beat again until fluffy and light. Beat in vanilla, flax egg, and red food coloring.
Making the Batter: Beat in about a third of the vegan buttermilk with the batter. Then beat in around half of the flour mixture.
Repeat, by beating in a third of the buttermilk, followed by half of the remaining flour. Finish by beating in the final third of the buttermilk.
Bake: Use a cookie scoop (or a 1/8 measuring cup) to make even scoops on the prepared pans, spacing them about 2 inches in between each. This should make around 40 cookies. Bake for 9 to 11 minutes. The little cakes should spring back when you gently press it with your finger.
Cool the cookies on the pan for 3 minutes, then transfer to a wire rack to cool completely.