Preheat the oven to 350°F and place the rack in the middle position.
In a small bowl, whisk together the flour, cornstarch, salt, baking powder, and baking soda. Set aside.
In the bowl of your stand mixer fitted with the beater attachment (or in a medium-sized bowl using your hand mixer), add the butter and both sugars. Cream on high for 2-3 minutes until the mixture is light and fluffy.
Next, add the egg and beat until incorporated. Then add the vanilla and beat an additional 30 seconds until smooth.
Reduce the mixer speed to low and add in the flour. Mix until almost completely mixed in, then add in ½ cup chocolate chips and beat until combined, about 1 minute. Set the remaining ¼ cup of chocolate chips aside.
Cover the bowl and place in the refrigerator for one hour until firm, but scoopable.
Scoop 6 cookie balls and place evenly in an 8-inch cast iron skillet. Press gently. Scatter remaining ¼ cup chocolate chips on top.
Bake for 18-20 minutes, then remove from the oven. Let sit for 5 minutes, then slice and serve.