In a small bowl, whisk together all-purpose flour, cocoa powder, baking soda, salt, and cornstarch.
In a mixing bowl, combine melted butter, brown sugar, white sugar, egg, egg yolk, and vanilla extract. Mix until smooth.
Add the dry ingredients to the wet mixture and combine until a dough forms, using a rubber spatula or a stand mixer with a paddle attachment.
Mix in the white chocolate chips and semi-sweet chocolate chunks. Cover the dough tightly and chill it in the fridge for 30 minutes.
Preheat the oven to 350°F.
Grease the skillet with oil spray or butter to prevent sticking.
Add the cookie dough to the skillet, filling it halfway to two-thirds full, as the cookies will rise.
Bake in the oven for about 12 to 14 minutes until the edges are golden and the center is still slightly soft. The pizookie will continue to bake out of the oven.
Top with scoops of vanilla ice cream and hot fudge sauce if desired, and enjoy!
Notes
Feel free to adjust the amount of chocolate to your liking. You can also use only white chocolate chips or swap them out for Hershey's cookies 'n cream chocolate and Oreos for a red velvet cookies 'n cream fusion.