1 1/2 sticks unsalted butter, softened to room temperature
1 cup packed light or dark brown sugar
1 large egg + 1 egg yolk1
2 teaspoons vanilla extract
2 cups all-purpose flour
2 teaspoons cornstarch
1 teaspoon baking soda
1/2 teaspoon salt
3/4 cup semi-sweet chocolate chips
Preheat oven to 350°F. Spray a 9-inch pie dish or cake pan with nonstick spray. Set aside.
In a large bowl using a hand-held mixer, beat the butter for 1 minute on medium speed until completely smooth and creamy. Add the brown sugar and beat on medium speed until creamy, about 1 minute. Mix in egg, egg yolk, and vanilla on medium-high speed until combined. Scrape down the sides and bottom of the bowl as needed.
In a separate bowl, combine flour, cornstarch, baking soda and salt. Add to butter mixture & stir until well combined. (The cookie dough will be quite thick). Add chocolate chips and stir until evenly disbursed.
Press the cookie dough evenly into the prepared pan. Bake for 30 minutes or until the cake is lightly golden brown. You may want to cover the cake loosely with aluminum foil after 15 minutes to avoid heavy browning around the edges. Remove from the oven and set the pan on a wire rack to cool completely. Once cooled, use a sharp knife or metal spatula to loosen the sides of the cookie cake from the pan and transfer to a serving dish. (Or use can serve directly from the pan). Cookie cake remains fresh covered tightly at room temperature for up to 3 days.