Preheat your oven to 350ºF. Line a baking sheet with parchment paper.
Cream butter with the brown and granulated sugars until smooth, about 2 minutes. Scrape bottom of bowl.
Add egg and vanilla extract and beat until well blended.
Add flour, cornstarch, baking soda and salt to creamed butter mixture and beat until just combined. Add chocolate chips and stir until just mixed in.
Drop scoops (about 1 tablespoon) of dough onto your baking sheet, spacing cookie dough about 2 inches (5 cm) apart.
Bake on the centre rack of your oven for 8-10 minutes, until cookies are golden around the edges.
Let cookies cool for a couple of minutes on the sheet before moving them to a rack to continue cooling.
Notes
Let unsalted butter soften just enough so that you can make an indent in it with your finger. If the butter gets too soft your cookies will spread and result in flat and crispier cookies.
When measuring your flour, make sure to spoon the flour into the measuring cup and then level it. Do not scoop it out directly of the bag with your measuring cup.
The cookies will keep well in a covered container at room temperature for up to 1 week. They can be frozen for up to 3 months in an airtight container.