Step 1

Preheat the oven to 350°F. Line a large baking sheet with parchment paper.

Step 2

In a small bowl, whisk together the flour, cornstarch, baking powder, and baking soda. Set aside.

Step 3

In the bowl of an electric mixer fitted with the paddle attachment, or in a large bowl with a handheld mixer, beat the butter, brown sugar, and granulated sugar together on high speed for about 5 minutes, scraping down the sides and bottom of the bowl. Beat until the mixture is lightened in color and there are no visible granules of sugar.

Step 4

Add the egg yolk and vanilla extract, and beat on high speed for another minute until fluffy.

Step 5

Add the flour mixture, and use a rubber spatula to gently mix it together just until there are no remaining streaks of flour. Do not over-mix the dough after the flour is added.

Step 6

Add the chocolate and stir just until combined.

Step 7

Split the dough into three portions, and roll into balls. Place the cookie dough balls at least three inches apart on the prepared baking sheet.

Step 8

Bake the cookies for 12 minutes, or until the outsides are golden brown and the insides still look a little gooey. The residual heat from the pan will cause the cookies to continue to bake a little after being removed from the oven, so don't over-bake them.

Step 9

The cookies are best within three hours of removing from the oven, but you can store them in an airtight container for up to a week.