Preheat your oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone mats.
In a small bowl, mix the ground flaxseed and water. Set aside for a few minutes to form your flax egg.
In a large bowl, use a hand mixer or stand mixer to beat the softened vegan butter and brown sugar until creamy, about 1–2 minutes.
Add the vanilla extract and the flax egg, mixing until well combined.
Stop the mixer and add the flour. Sprinkle the cornstarch, baking soda, and salt over the flour. Mix on low speed just until the dough forms.
Gently fold in the non-dairy chocolate chips or chunks.
Roll the dough into balls using about 1–2 tablespoons per cookie. Place them on the prepared sheets, leaving space in between.
Bake for 10 minutes (12 minutes for larger cookies), until the edges are slightly golden. Avoid overbaking to keep them chewy.
Let the cookies cool on the baking sheet for 5 minutes before transferring them to a cooling rack.