Prep: Preheat the oven to 350F. Spray or line a large baking tray with parchment paper or a silicone mat and set aside. Prepare the flax “egg” by mixing the flaxseed and water well; set aside for at least 5 minutes to let thicken.
Cream the Butter: Add the softened butter, cane sugar, and brown sugar to a large bowl or stand-mixer. Use a stand mixer or electric mixer to cream the butter and sugar together on low speed for 3 to 5 minutes, until the butter is smooth, light, and fluffy. Scrape the sides of the bowl with a spatula as necessary.
Wet Ingredients: Add the flax “egg” and vanilla to the bowl and beat again, until the mixture is smooth and combined.
Dry Ingredients: Add the flour, cornstarch, baking soda, baking powder, and salt. Mix well until a thick, non-sticky dough forms. Add the chocolate chips to the batter and fold into the dough using a spatula.
Form the Cookies: Use a cookie scoop or spoon to form 12 cookies with ~3 tablespoons of dough per cookie. Roll each cookie into a smooth ball and space evenly across the prepared baking sheet. Do not flatten the cookie dough; it will spread as it bakes in the oven.
Bake: Bake in the middle rack of the oven for 10 to 12 minutes; the cookies will look puffy and pale, but the bottom edges should be golden.
Cool: Take the cookies out of the oven and sprinkle with flaky salt, if desired. Let the cookies rest on the baking sheet for another 10 minutes, where they will deflate further. Transfer to a wire cooling rack and let cool completely.
Serve & Store: Enjoy the cookies immediately, or store in an airtight container for up to 5 days. You can also store the baked cookies in the freezer for up to 2 months, but I would recommend storing the cookie dough balls instead and baking when desired :)