Preheat the oven to 350 degrees Fahrenheit. Line two baking sheets with parchment paper or silicone baking mats.
In a small bowl, mix together the ground flaxseed and water. Set aside for a minute to thicken into a flax egg.
In a large bowl using a handheld mixer or a stand mixer with the paddle attachment, beat the softened vegan butter and packed brown sugar together for 1 to 2 minutes until creamy and well combined.
Add the vanilla extract and the prepared flax egg. Mix until fully incorporated.
Stop the mixer and add the all purpose flour to the bowl. Sprinkle the cornstarch, baking soda, and salt evenly on top of the flour. Turn the mixer on low speed and mix until just combined. Do not overmix.
Add the vegan chocolate chips and stir them in either by hand with a spatula or using the mixer on the lowest setting.
Roll the dough into balls, about 1 to 2 tablespoons each. Place them on the prepared baking sheets, leaving space between each cookie for slight spreading.
Bake for 10 minutes for smaller cookies, or 12 minutes for larger ones, until the cookies are barely golden around the edges. They will look puffy and lightly colored in the center. Do not over bake.
Remove from the oven and let the cookies cool on the baking sheet for 5 minutes. They will continue to set up during this time. Transfer to a wire cooling rack to cool completely. The cookies will firm up and become perfectly chewy as they cool. Enjoy!