Preheat the oven to 350F/177C. Line two baking sheets with parchment paper or silicone baking mats. If you plan on chilling your cookies, save this step for after the cookies have chilled.
Mix together the flour, baking soda, salt, and cornstarch, in a medium bowl and set aside.
Beat the butter and both sugars together until light and fluffy, about 3 - 4 minutes on medium speed in a mixer. Beat in the egg plus yolk and then add the vanilla and beat well again for about another minute.
Fold in the flour until just combined. Fold in the chocolate chips (reserve some to stud the top of the cookies).
(Optional) Chill the dough for an hour. You can skip this if you really want your cookies now, but you get a better flavor when you chill.
Scoop out balls of cookie dough at least 3 tablespoons bit and roll into balls. Place them at least 2 inches apart on your baking sheets.
Bake for 10 - 14 minutes, or until the edges are a golden brown and the tops still look underbaked. Remove and top with chocolate chips (you will need to carefully push them in so they stick). Leave them on the baking sheet for at least two minutes before transferring to a wire rack to cool completely.
Store uneaten cookies in an airtight container for up to a week.
Notes
Always make sure your ingredients are at room temperature. You can quickly bring your eggs to room temperature by placing the whole eggs in warm water for five minutes.
You can use arrowroot starch instead of cornstarch, or omit it completely if you don't have it. You will still have soft, thick cookies.
Spoon and level your flour when measuring, or better yet, use a kitchen scale.
The recipe doubles very well if you want to make extra for later.
You can experiment with different types of add-ins. Nuts, candies, dried fruit, or even omit the chocolate chips altogether.