Preheat the oven to 350°F (175°C). Line two baking sheets with parchment paper or silicone baking mats.
In a small bowl, stir together the ground flaxseed and water. Set aside to thicken for 5 minutes - this is your flax egg.
In a large bowl, beat the softened vegan butter and brown sugar using a hand mixer or stand mixer for 1–2 minutes, until light and creamy.
Add the vanilla extract and flax egg to the butter-sugar mixture. Mix until fully combined.
Add the flour to the bowl. Sprinkle the cornstarch, baking soda, and salt directly on top of the flour. Mix on low speed until just combined - do not overmix.
Fold in the chocolate chips (and chunks if using) until evenly distributed.
Scoop the dough into balls of about 1–2 tablespoons each. Roll lightly with your hands and place on the prepared baking sheets, spacing them a couple inches apart.
Bake for 10 minutes (12 minutes for larger cookies), until the edges are barely golden. The centers will look slightly underdone - that's correct! Do not overbake.
Allow cookies to cool on the baking sheet for 5 minutes, then transfer to a wire cooling rack. They will firm up perfectly as they cool. Sprinkle with flaky sea salt if desired.