In a medium bowl, whisk the flour, oats, baking soda, baking powder, cornstarch, and salt together. Set aside. 2 ¼ cups (270 g) all-purpose flour ¾ cup (75 g) old fashioned oats 1 teaspoon baking soda ½ teaspoon baking powder 1 teaspoon cornstarch ½ teaspoon salt
In a large bowl with an electric mixer cream the softened butter, dark brown sugar, and granulated sugar together until light and fluffy, about 2 to 3 minutes on medium speed. Scrape down the bowl as needed. I like to use my stand mixer fitted with the paddle attachment but you can also use a hand mixer (creaming will take a bit longer.) ¾ cup (170 g, 1 ½ sticks) unsalted butter ¾ cup (150 g) packed dark brown sugar ½ cup (100 g) granulated sugar
Add in the egg and egg yolk and beat to combine. Scrape down the bowl. Add in the vanilla extract and beat to combine. 1 large egg 1 large egg yolk 2 teaspoons pure vanilla extract
With a spatula or wooden spoon, stir in the flour mixture and mix together. The dough will be soft and thick. Fold in the chocolate chips. ¾ cup (127 g) semi-sweet chocolate chips ¾ cup (127 g) milk chocolate chips
Using a large cookie scoop that holds ⅓ cup scoop the dough (scoop #12). Roll into balls and place into a parchment lined baking sheet.
Cover the unbaked cookies tightly with plastic wrap and chill in the refrigerator overnight for at least 2 hours, ideally overnight. Alternately, you can chill the bowl of cookie dough and scoop the next day (just allow several minutes at room temperature to allow for easy scooping).
When ready to bake, preheat oven to 350°F (180°C). Line large baking sheets with parchment paper or silicone baking mat. Place cookie dough balls spaced a few inches apart.
Bake the cookies for 11-13 minutes or until the edges are very lightly browned. The centers will look very soft, but the cookies will continue to set as they cool. When they come out of the oven I like to press a few extra chocolate chips on top for appearance! Cool on the baking sheet for 10 minutes. After 10 minutes of cooling on the baking sheets, transfer cookies to a wire rack to cool completely. Cookies stay fresh covered at room temperature for up to 1 week.