Add the softened butter, brown sugar, and granulated sugar to a large mixing bowl. Beat on medium speed for about 2–3 minutes, until the mixture looks creamy, smooth, and slightly lighter in color. Scrape down the sides of the bowl as needed so everything mixes evenly.
Mix in the egg and vanilla until fully incorporated. Mix until the dough looks smooth and evenly combined.
In a separate bowl, whisk together the flour, baking soda, cornstarch and salt. Add the dry ingredients to the butter mixture gradually, mixing on low speed or folding gently with a spatula. Stop mixing as soon as you no longer see streaks of dry flour. Overmixing can make the cookies less tender.
Add the chocolate chips and gently fold them into the dough. Try not to overwork the dough at this point; you just want the chocolate evenly distributed throughout.
Cover the dough and let it chill in the refrigerator. While chilling isn’t necessary for this recipe, it can help the cookies hold their shape and gives the flavors a little more time to develop. If you’re short on time, you can skip this step and bake the cookies right away.
Scoop the dough using about 2 tablespoons or a medium cookie scoop (approximately 30g per cookie). Place the dough balls onto a parchment-lined baking sheet, leaving enough space between each one for the cookies to spread as they bake.
Bake in a preheated oven at 350°F (175°C) for 9–10 minutes, until the edges are lightly golden and set while the centers still look soft.
Let the cookies sit on the baking sheet for a few minutes after baking. Then transfer them to a wire rack to cool completely. They’ll firm up as they cool but stay soft and chewy inside.