Preheat the oven to 350°F. Lightly grease a 10-inch cast-iron pan or other oven-safe pan. If you have neither, grease a cookie sheet or line it with parchment paper.
In a medium bowl, whisk together the flour, cornstarch, baking soda, and salt. Set aside.
In a large bowl, use a mixer to beat the butter at medium-high speed for about a minute, or until the butter is smooth and fluffy. Add the brown sugar and granulated sugar and beat for another minute or two, until the mixture is creamed. Then, add the eggs and vanilla extract and beat for a minute, scraping down the sides of the bowl as needed.
Add the dry ingredients to the wet mixture in thirds. Mix on low speed until combined. Then, fold in the semi-sweet chocolate chips.
Press the cookie dough into the pan (an easy step for little ones with clean hands).
Bake the giant cookie for up to 25 minutes, or until the edges are golden brown. The middle should still be soft and gooey.
Carefully remove the pan from the oven and allow it to cool for 5 minutes, minimum. Add toppings and serve. Yum!
