Preheat oven to 350 degrees. Spray 8 or 9-inch cake pan or springform pan with nonstick cooking spray.
In a large mixing bowl, cream butter, brown sugar, and sugar for 3-4 minutes scraping the sides of the bowl often, until light and creamy.
Add egg, egg yolk, and vanilla. Mix for 1 minute.
Stir in flour, cornstarch, baking soda, and salt, just until combined. Stir in chocolate chips.
Spread cookie dough into cake pan.
Bake for 20-26 minutes, depending on the depth of the pan. If using a large, thin pizza pan, reduce baking time for 15-18 minutes. If the cookie is browning too much, cover with foil halfway through baking time.
Check for doneness by inserting a toothpick in the center of the baking pan. Set aside to cool.
Once it is cooled, run a knife along the sides of the baking pan. Invert onto flat, larger serving platter or plate. May use a heavy object, such as a rolling pin, to bang on the bottom of the pan, once it is inverted to help release the cookie from the pan.
To make frosting: cream butter for 2 minutes. Add cocoa powder (if making chocolate frosting), powdered sugar, vanilla, and cream.
Using a cake decorator set, frost the cooled cookie, according to preference.
Cut into wedges to serve.
Notes
-- If you are doing a lot of decorating, I would suggest doubling the frosting recipe to ensure you have enough.