Preheat oven to 375°F. Line a baking sheet with parchment paper or a silicone baking mat.
In a medium bowl whisk the flour, cornstarch, baking powder, baking soda, and salt.
Cream the butter, brown sugar, and granulated sugar on medium speed (with paddle attachment) for 2-3 minutes, scraping down the sides of the bowl as needed.
Add the egg and vanilla, beat on medium speed for 1 minute.
Add the dry ingredients and mix until JUST combined. Stir in the chocolate chips.
Using a ½ cup measuring cup, portion the dough into equal 1/2 cup size balls (I like to use a kitchen scale to get the balls equally portioned. Each dough ball should weigh 5-5.5 oz each). See note #2. Place the dough balls on the prepared baking sheet. Leave enough space between each cookie, I do 2 per tray.
Bake for 9-12 minutes. The tops will be lightly golden and the tops set. Remove the baking sheet from the oven and allow the cookies to sit on the tray for 10 minutes (the cookies will continue to bake/set while on the sheet). Transfer to a wire rack to cool for at least 10 minutes before eating. The cookies need time to set. (Refer to the notes section for more tips).
Store cookies in an airtight container. See note #3