Preheat the oven to 375°F/190℃. Line 2 baking sheets with parchment paper. Set aside.
Make the flax egg. In a small heat proof bowl, mix the ground flax seeds with boiling water. Mix well and set aside for 5 minutes to gel.
Mix the wet ingredients. In a large bowl with your hand mixer, or in a stand mixer with the paddle attachment, mix the butter with white and brown sugars on medium speed for 5 minutes. Add the flax egg mixture and vanilla.
Mix the dry ingredients. In a medium sized bowl, sift together the dry ingredients: flour, baking soda, cornstarch, salt, and cinnamon.Now add the dry ingredients, mixing on low speed, and making sure to scrape down the sides of the bowl to ensure all the ingredients are mixed. Add the chocolate chips (or chopped chocolate). Let it mix together one last time.
Transfer. Scoop cookie dough onto the prepared baking trays using a 1 tablespoon cookie scoop, or using a tablespoon. Sprinkle with the flaky salt if using.
Bake. Bake for 12-15 minutes or until the cookies are golden brown and have crisp edges. Remove from the oven and let rest on the hot cookie tray for 10 minutes before removing to a wire rack to finish cooling. Enjoy!
Notes
How to Store
These cookies are best eaten in 5 days if kept in an airtight container on the countertop.
To store them before baking, make the cookie dough and portion out into dough balls. Freeze the cookie dough for 1 hour on a cookie sheet, then transfer to a freezer safe container. These will keep frozen for up to 3 months. To bake from frozen, add an additional 3-5 minutes of baking time.
To store them after baking, place fully cooked and cooled cookies into a freezer safe container. Layer parchment paper or wax paper in between each layer of cookies. This will keep for up to 3 months. Make sure you allow them to thaw completely before enjoying from frozen.