Preheat oven to 325°F. Spray 8 or 9 inch spring form pan with non stick 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 and mix for 1 minute.
Stir in flour, cornstarch, baking soda, and salt mix until just combined. Stir in the chocolate chips.
Spread the cookie dough in the spring form pan.
Bake for 30- 35 minutes. Check for doneness by interesting a toothpick in the center of the baking pan. Set aside to cool.
For frosting: Beat butter, corn syrup, vanilla in a large mixing bowl until light and creamy. Gradually add powdered sugar and milk until fluffy, adding additional milk as needed to reach desired spreading consistency.
Once cookie cake is cooled, run a knife along the side of the baking pan. Pipe frosting generously onto cookie cake with a pastry bag and decorative tip. Note: If spreading frosting on cakes with a knife (instead of piping), use a half recipe of the frosting.