Preheat oven to 350 degrees F. Liberally grease a 9" springform pan with cooking spray and set aside.
In the bowl of a stand mixer, cream together the butter, brown sugar and sugar until blended, about 1 minute. Add in the egg, egg yolk and vanilla and beat until combined. Lastly, add in the salt, baking soda, cornstarch and flour and beat until a soft dough has formed. Fold in the chocolate chips.
Press the mixture into the prepared pan in an even layer. Bake for approx. 20-25 minutes or until the top is light golden brown and the center appears mostly set. If it's still gooey looking, that's okay - you do not want to over bake. But if it's sloshing around in the pan, bake it for another 5-10 minutes if needed. Cool in the pan completely, then gently release the cookie cake from the pan.
To make the frosting, in a large bowl, beat together the butter, cocoa powder and vanilla until smooth. Add in the powdered sugar and cream and beat until smooth and creamy. Pipe around the perimeter of the cookie cake and immediately top with a sprinkling of mini chocolate chips or chocolate sprinkles. Serve!