Whisk the flour, cornstarch, baking soda, and salt together in a medium bowl. Set aside.
In a large bowl, whisk the butter, brown sugar, and granulated sugar together until combined and no brown sugar lumps remain. Whisk in the egg and egg yolk, then whisk in the vanilla extract until combined.
Add the dry ingredients to the wet ingredients and stir with a rubber spatula or wooden spoon until the cookie dough comes together. Stir in the chocolate chips. The dough will be very soft and even a little wet. Cover and chill the dough in the refrigerator for at least 2 hours (and up to 4 days). If chilling for longer than 3 hours, allow it to sit at room temperature for at least 30 minutes before rolling and baking the dough will be quite hard.
Preheat oven to 325 degrees F. Line baking sheets with parchment paper or silicone baking mats. Set aside.
Roll balls of dough, 2 scant tablespoons of dough per cookie, and place 3 inches (7.5 cm) apart on the baking sheets. Bake for 12 to 13 minutes, or until lightly browned on the sides. The center will look soft.
Remove from the oven and allow to cool on the baking sheets for 5 minutes before transferring to a wire rack to cool completely.
Cookies will stay fresh in an airtight container at room temperature for up to 1 week.
Notes
The original recipe called for regular semisweet chocolate chips, but I substituted mini chocolate chips. Either would be delicious!
I used a medium cookie scoop for portioning out the dough, which is approximately 2 tablespoons, so these might be just a teeny tiny smidge larger than the scant 2 tablespoons called for in the recipe.