In a large bowl using a handheld mixer or a stand mixer fitted with a paddle attachment, beat the butter, brown sugar, and sugar together on medium speed until combined and creamy, about 2 minutes. Beat in the egg and vanilla. Scrape down the sides and bottom of the bowl as needed.

In a separate bowl, whisk flour, cornstarch, baking soda, and salt together. Add into the wet ingredients, then beat on low speed until combined. The cookie dough will be slightly thick. On low speed, beat in the chocolate chips.

Cover dough tightly and chill in the refrigerator for at least 1 hour and up to 3–4 days. Chilling is imperative for this cookie dough.

Remove cookie dough from the refrigerator and allow to sit at room temperature for 10 minutes. Preheat oven to 350°F (177°C). Line 2 large baking sheets with parchment paper or silicone baking mats. Set aside.

Once chilled, the dough will be slightly crumbly, but will come together when you work the dough with your hands. Roll cookie dough, about a heaping 1.5 Tablespoons of dough per cookie (I use this medium-size cookie scoop), and place 3 inches apart on baking sheets.

Bake for 11–12 minutes, until barely golden brown around the edges. The cookies will look extremely soft when you remove them from the oven. Cool for 5 minutes on the baking sheet. If the cookies are too puffy, try gently pressing down on them with the back of a spoon. They will slightly deflate as you let them cool. If desired, while the cookies are still warm, press a few extra chocolate chips into the tops. This is optional, just for looks.

Transfer cookies to a cooling rack to cool completely. Cookies stay fresh covered at room temperature for up to 1 week.