Preheat the oven to 350ºF. Spray a 9" springform pan with non-stick spray*. Set aside.
In a large bowl, toss together the flour, cornstarch, baking soda, and salt. Set aside.
In a medium size bowl, whisk together the butter, brown sugar, and granulated sugar until smooth. Whisk in the egg, egg yolk, and vanilla extract until completely combined.
Pour the wet ingredients into the dry ingredients and stir until no flour clumps remain. Fold in the chocolate chips. Press the cookie dough evenly into the prepared pan. Bake for 23-25 minutes until edges begin to brown. You may cover the cake with foil to prevent browning, if preferred. Remove from oven and allow to cool on a wire rack completely before decorating with frosting.
CHOCOLATE FUDGE FROSTING
In a small saucepan over medium heat, melt the unsweetened baking chocolate and the butter. Allow to cool for 15 minutes.
In a medium bowl with a handheld mixer or the bowl of a stand mixer fitted with the paddle attachment, combine cooled chocolate/butter mixture, vanilla extract, and milk or cream (starting with 3 Tablespoons). Turn the mixer onto low and slowly add the powdered sugar. Increase mixer speed to medium and beat until completely combined, scraping down sides as needed. Add salt to taste.
Remove cooled cookie cake from springform pan and place on serving platter. Fill a piping bag with the frosting and decorate the cookie cake. I like to use a Wilton 1M tip. If using, add sprinkles before frosting sets. Cookie cake remains fresh covered tightly at room temperature for up to 4 days. Undecorated cookie cake may be frozen, up to 2 months. Thaw in refrigerator overnight and allow to come to room temperature before decorating.
*Alternatively, you may use a 9" pie plate or a 8" or 9" round or square baking pan.