Step 1

Preheat oven to 325°F (163°C). Line 3 large baking sheets with parchment paper or silicone baking mats. (If you don’t have 3 baking sheets, bake the cookies in batches.)

Step 2

Whisk the flour, cornstarch, baking soda, and salt together in a medium bowl.

Step 3

Using a handheld or stand mixer fitted with a paddle attachment, beat the butter, brown sugar, and granulated sugar together in a large bowl on medium-high speed until combined and creamy, about 2 minutes. Add egg, egg yolk, and vanilla extract, then beat on medium-high speed until combined. Scrape down the sides and up the bottom of the bowl as needed. Beat in the dry ingredients on low speed, then beat in the chocolate chips until combined. The dough will be thick.

Step 4

Divide dough into 6 portions. Each will be a heaping 1/2 cup and weigh a little over 6 ounces. Shape each into mound, slightly flattening down the top. Place two on each baking sheet, at least 6 inches apart. Bake for 20-25 minutes or until edges and top are lightly browned. The center will look super soft, but it will set as the cookie cools.

Step 5

Cool the cookie on the baking sheet for 15 minutes, then transfer to a wire rack to cool completely. As it cools, press a few chocolate chips into the top, if desired, for garnish.

Step 6

Cookies stay fresh covered at room temperature for up to 1 week.