Preheat oven to 350° and line an 8 or 9 inch springform pan or round cake pan with foil. Spray with non stick spray. Set aside.
Beat together butter, brown sugar, and sugar, followed by vanilla and egg until smooth.
Beat in flour, cornstarch, baking soda, salt and milk until thick.
Place in the oven and bake for 22-25 minutes or until golden brown.
Carefully remove from oven and let cool.
Beat powdered sugar, heavy cream, butter, vanilla and gel color together.
Pipe onto outer edge of cookie cake as desired, add more sprinkles, and enjoy!
Recipe mostly adapted from Life, Love and Sugar