Preheat the oven to 350℉.
In a 1 Qt. mixing bowl, using a hand mixer, cream the butter and sugars on medium speed for about 1 minute.
Add the egg yolk and vanilla and mix until all is combined.
In a small bowl, whisk the flour, baking soda, cornstarch, and salt together.
Mixing on low speed, add the dry ingredients to the wet mixture. Mix until combined. Add the chocolate chips and mix with a spoon.
Chill the cookie dough for 15 minutes.
Using your cookie scoop, place heaping 2 tablespoons of batter on the prepared cookie sheet, leaving 1 inch between cookies. You should have 6 cookies.
Bake for about 12 - 15 minutes. Cookie edges should be light brown. The centers may look slightly underbaked. They will continue to cook on the cookie sheet.
Remove from oven and let the cookies stay on the cookie sheet for 5 minutes. Transfer to a wire rack to finish cooling.