In an electric mixer fitted with the paddle attachment (or the whisk), combine the butter with the granulated and brown sugar. Mix on medium speed for 5 minutes. Add the whole egg and beat 5 minutes more. Add the egg yolk and beat for 2 minutes, or until the batter is smooth. Use a rubber spatula to scrape down the sides of the bowl if necessary.

Meanwhile, in another bowl, whisk the flour, cornstarch, and baking soda to blend them. Add the flour mixture to the butter mixture and beat just until the butter absorbs the flour. Don't overwork the dough; it's fine if there are still some visible streaks of flour.

Add the chocolate chips and mix for 2 minutes, or until they are combined.

Shape the dough into a large flat rectangle. Wrap in plastic wrap and refrigerate overnight. Before using the dough, let it sit on the counter for at least 30 minutes until it's pliable enough to shape.

Set the oven at 350 degrees. Line 2 baking sheets with silicone mats or parchment paper.

Divide the dough into even-sized balls, each about 50 to 55 grams (about ¼ cup). Roll them tightly in your hands. Set them on the baking sheets in offset rows, no more than 8 per sheet. Bake the sheets one at a time for 12 minutes, or until lightly golden. The top of each cookie should still look slightly gooey and soft, with golden edges. Transfer to a wire rack to cool. Store in an airtight container for up to 1 week.

Note: Store unbaked cookie balls in a zipper plastic bag in the freezer for up to 1 month. When you bake the cookies (without defrosting), add 3 minutes to the baking time.