Melt the butter in a microwave-safe bowl and let it cool to room temperature.

In a large bowl, mix the cooled butter with the sugars and whisk until combined. Add the egg and vanilla, then whisk until smooth and emulsified.

Whisk in the cornstarch, baking soda, baking powder, and salt. Stir in the flour until just combined, being careful not to overmix.

Fold in 1-1/4 cups of chocolate chips. Divide the dough into 8 equal portions (4 ounces or 113 grams each) and chill in an airtight container for 30 minutes.

Preheat the oven to 325?F. Line a sheet pan with a silicone baking mat.

Place 3 dough balls on the sheet pan, giving enough space for spreading.

Bake for 13-18 minutes, slightly under-baking for a soft and chewy texture. The cookies will continue to bake a little after being removed from the oven.

Press the remaining chocolate chips onto the tops of the cookies. Let cool on the tray for 10-15 minutes, then transfer to a cooling rack. Optionally, sprinkle with flaky sea salt.