Preheat the oven to 350 degrees F and lightly mist a 10-inch tart pan with non-stick spray.
Place the butter, brown sugar and granulated sugar in a large mixing bowl and beat on medium-high speed until very pale and fluffy (3 to 5 minutes).
Add the egg, egg yolk, and vanilla, beating until completely incorporated.
Scrape the bottom and sides of the bowl with a silicone spatula, then add the flour, cornstarch, baking soda, and salt, stirring on medium-low speed just until all the ingredients are combined (do not over-mix).
Stir in the chocolate chips.
Transfer the cookie dough to the prepared pan, spreading it in an even layer.
Bake for 22 to 28 minutes, or until golden around the edges but still soft in the middle.
Cool completely, then decorate with buttercream and rainbow sprinkles.
To decorate the chocolate chip cookie cake:
Place the butter and salt in the bowl of an electric mixer, and beat on medium speed until smooth.
Add the powdered sugar, a half cup at a time, beating on medium speed until incorporated.
Stir in the vanilla extract.
Whip the frosting on medium-high speed until very fluffy.
Whip in the cream (as needed) to achieve a more spreadable consistency.
Transfer the buttercream to a piping bag fitted with a jumbo open star tip, and pipe swirls around the outside edge of the cooled chocolate chip cookie cake.