Preheat oven to 350°F. Place a sheet of parchment paper on the bottom portion of a springform pan and lock in the sides. Grease sides of springform pan with non-stick baking spray. (You can also use a 9-inch baking dish or pie dish. Be sure to spray it with non-stick baking spray.) Set aside.

In a large bowl, beat butter, brown sugar and granulated sugar until light and fluffy, about 2-3 minutes.

Add in egg, vanilla extract, baking soda and salt and beat to combine. Then add cornstarch and flour, mixing until well combined. Fold in chocolate chips.

Press the cookie dough evenly into your baking pan. For a prettier appearance, I like to sprinkle a few chocolate chips on top and press them into the the top of the dough.

Bake cookie cake on the center rack of your oven for 21 to 23 minutes. You do not want to over bake this cookie cake! Remove from oven once the edges are starting to turn golden and the center is set. If your edges are cooking too fast, you can always cover the outside edges with foil (leaving the center uncovered, to prevent the edges from browning too much).

Allow the cookie cake to cool on the counter before decorating or trying to remove from the pan.

Once cooled, transfer the cookie to a serving platter and decorate with frosting, if desired.

Using a stand mixer or hand mixer, beat butter, cocoa powder, vanilla and salt together until smooth in a large bowl. Add powdered sugar in two additions, beating until smooth each time, scraping the sides of the bowl as needed.

Slowly beat in milk, a tablespoon at at a time, until frosting reaches desired consistency. Frosting should be light and fluffy and easily spreadable.

Frost on top of cookie cake as desired!