In a small bowl, whisk together the ground flaxseed and water. Let it sit for 5 to 10 minutes until it becomes thick and gel-like.
In a large mixing bowl, beat the softened vegan butter, cane sugar, and brown sugar together using a hand mixer or stand mixer on medium speed. Continue until the mixture is light, fluffy, and pale in color, about 3 to 5 minutes.
Mix in the vanilla extract and the prepared flax egg until fully combined.
In a separate bowl, sift together the all-purpose flour, cornstarch (or arrowroot powder), baking soda, baking powder, and kosher salt.
Gradually add the dry ingredients to the wet mixture, folding gently with a spatula until just combined. Avoid overmixing to keep the cookies tender.
Stir in 1 to 1 1/2 cups of dairy-free chocolate chips or chunks.
Preheat your oven to 350°F (175°C). Using a cookie scoop or tablespoon, drop dough balls about 2 inches apart onto the prepared baking sheet. Sprinkle a pinch of flaky salt on top of each cookie before baking.
Bake for 10 to 12 minutes, or until the edges are golden but the centers still look slightly soft. The cookies will firm up as they cool.
Allow the cookies to cool on the baking sheet for 5 minutes before transferring to a wire rack to cool completely. Enjoy warm or store for later.