In a large bowl or stand mixer bowl with the paddle attachment, beat the butter on medium speed until light, smooth, and creamy about 1 minute.
Add in the brown sugar and white sugar, and mix on medium-high speed until fully incorporated, light in colour and fluffy.
Blend in the egg and vanilla, scraping the sides of the bowl as needed.
In a separate medium bowl whisk or sift together the flour, cornstarch, baking soda, and salt.
Slowly mix the flour mixture into the wet ingredients until combined. The dough will be fairly thick. If using a stand mixer remove the paddle and bowl.
Using a wooden spoon mix in the chocolate chips, ensure they are evenly mixed in.
Cover the dough surface completely with tin foil, plastic wrap, or bees wax wrap and set in the fridge for a minimum of 2 hours up to 2 days. Chilling is required and is not optional.
Remove the dough from the fridge and allow it to sit for at least 10 minutes, if the dough was chilled for longer than 2 hours allow it to sit for about 30 minutes.
Preheat the oven to 350 °F, and line two large cookie sheets with parchment paper or non-stick baking sheets.
Scoop and roll the dough into balls approximately 1 Tbsp, I usually use an ice cream/cookie scoop which is about 2 Tbsp.
Place 10-12 on each baking sheet at least 2 inches apart.
Bake for 8-10 minutes, until the edges are barely golden, they will look incredibly soft and puffy.
Allow them to cool on the sheet for 5 minutes, they will deflate slightly as they cool. Transfer to a cooling rack to cool completely.