Melt the butter in a microwave-safe bowl. Let it cool to room temperature—hot butter can make the cookies greasy.
Once butter has cooled to room temperature, use a spatula to scrape it all into a large bowl. Mix in the sugars and whisk until combined. Add the egg and vanilla, then whisk briskly until smooth and emulsified.
Whisk in the cornstarch, baking soda, baking powder, and salt. Stir in the flour with a wooden spoon or hand mixer until just combined—don’t overmix.
Use a spatula to fold in 1-1/4 cups of chocolate chips. Divide dough into 8 equal portions (4 ounces or 113 grams each). Chill dough balls in an airtight container for 30 minutes.
Preheat oven to 325°F. Line a sheet pan with a silicone baking mat.
Place 3 dough balls at a time on the sheet pan, giving cookies plenty of space to spread.
Bake 13-18 minutes, erring on the side of slightly 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 chocolate chips on top. Let the cookies cool on the tray for 10–15 minutes, then transfer to a cooling rack. Sprinkle with flaky sea salt if desired. Repeat with the rest of the cookies.
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Notes
Storage: Store cookies in an airtight container at room temp for up to 5 days and microwave for 10–15 seconds to rewarm. To freeze dough, shape into balls, freeze until firm, then transfer to a freezer bag for up to 3 months. Bake straight from frozen, adding 1–2 minutes to the bake time.