Preheat the oven to 350F. Line two baking sheets with parchment paper.
Place the butter, brown sugar, and granulated sugar in a large bowl. Use a stand mixer or hand mixer to mix until well combined, about 2 minutes.
Add the egg and vanilla extract and mix until combined, and lighter in color, about 1 minute.
Add the flour, cornstarch, baking soda, and salt. Mix on low until just combined, about 30 seconds. Add the chocolate chips and mix until just combined.
Use a large (1/4 cup) cookie scoop to scoop the dough into balls. Place a few extra chocolate chips on top of each dough ball.
Place up to 5 dough balls on each baking sheet, spaced equally apart. Bake for 9-11 minutes, until the edges are just set. Top each cookie with a pinch of flaky salt (optional). Let the cookies cool on the baking sheet for 10 minutes, then transfer to a cooling rack to cool completely.