Melt the butter in a very large, microwave-safe bowl. Once melted, refrigerate for 5 minutes. It's important the butter isn't hot when you add in the sugars or it will melt the sugar and make the cookies flat/greasy.
Once butter is cooled to room temperature, mix in the light brown sugar and white sugar. Stir until smooth. Add in the egg and vanilla extract. Stir again until smooth.
Add in the cornstarch, baking soda, baking powder, and salt. Again, stir until smooth. Add in the correctly measured (See Note 1) flour and mix until just combined. Don't overmix.
Gently stir in 1 and 1/4 cups of chocolate chips until combined. Separate the dough into 4 equal parts. If you have a food scale, each ball should be just a little over 6 ounces (170g). Place those balls of dough in an airtight container and chill for 30 minutes to 1 hour.
Preheat the oven to 325 degrees F. Line a pan with parchment paper or use a nonstick liner.
Place two dough balls at a time on a sheet pan giving the cookies plenty of space to spread.
Bake for 20-25 minutes, erring on the side of under-baking, which keeps them soft and chewy. The cookies will "bake" a little more out of the oven.
Remove from the oven and press the remaining 1/4 cup chocolate chips onto the tops of the cookies (ensures even placement of chocolate and also makes them look pretty!). Allow cookies to cool on the baking tray for 10-15 minutes before transferring them to a cooling rack. If you like salty-sweet flavors, add a few flakes of salt on top!
STORAGE: We like these cookies best the day they're baked. They do last up to 1 week, but they begin to lose texture and flavor. To store: Place in an airtight container and keep at room temperature. Wait until cookies are completely cooled before adding to the container. Rewarm in increments of 10-15 seconds in the microwave.
Note 1: Spoon the flour into a measuring cup and level the top with the back of a table knife.