Preheat oven to 410 degrees F. Line a baking sheet with parchment or silicone mat.
Make flax egg in your mixer bowl by combining the ground flaxseed and water. Let it sit for 5 minutes to thicken. Add the butter, brown sugar, and sugar. With the paddle attachment, cream until well combined, about 2 minutes.
Add the flour, cornstarch, baking powder, baking soda, and salt. Mix dough until just combined. Add the chocolate chips and walnuts. Pulse dough until combined. Then use a spatula and gently mix the dough to make sure everything is evenly incorporated.
Refrigerate the dough for 30 minutes. I highly recommend this step, especially if your kitchen is a little warmer. You can bake right away, but they might spread a little more.
Divide dough into 6 equal, tall mounds (mine were about 4.9 oz each and about 2 inches tall). Don’t use a cookie scoop. You want it to be a rough/rustic mound. Do not flatten the dough.
Bake for 9-11 min or until lightly golden on the edges/tops and the centers are slightly set. Don’t over bake, they are supposed to be gooey inside. If your cookies are smaller, bake for less time. Let them cool on the pan for 20 minutes. Carefully remove and place on a wire rack to cool completely. Enjoy!