Preheat oven to 350°F/175°C. Line two cookie sheets with silpat or parchment paper.
Make the flax egg and set it aside.
Place the butter in a mixing bowl. Use either a handheld or stand mixer on medium speed and beat until light and fluffy. Add the sugars and beat again until light and fluffy. Finally, add the flax egg, orange zest, and vanilla and beat again to combine.
In a separate bowl, combine the flour, cornstarch (see note), baking soda, baking powder, and salt. Stir to combine. Pour the flour mixture in with the butter mixture. Stir to combine.
Add the chocolate chips and stir until combined. Be sure to cover the cookie dough when not in use to prevent it from drying out.
Drop by rounded spoonful about 2 inches apart on prepared cookie sheet. (Or you can roll the cookie dough balls for more precise edges to the cookies). Bake for 9 minutes (or up to 11 minutes, depending on your desired final cookie texture). If you prefer chewier cookies, remove them at 9 minutes. If you prefer crispier, cook for a few minutes longer. See note.
Once the cookies are done, allow them to cool for a minute or two on the cookie sheet before transferring to a wire rack to cool completely.
Store in a sealed container for up to a week, either on the counter or in the fridge. Or you can freeze these cookies for up to two months.