In a small bowl, whisk together all purpose flour, cocoa powder, baking soda, salt, and cornstarch.
In a mixing bowl, add the butter, brown sugar, white sugar, egg and egg yolk, and vanilla extract. Whisk together until smooth.
Add the dry to the wet and combine until a dough forms using a rubber spatula or a stand mixer fitted with a paddle attachment.
Mix in the white chocolate chips and semi sweet chocolate chunks. Cover tightly and pop in the fridge to chill for 30 minutes.
Preheat oven to 350°F.
Spray the skillet with oil spray or butter it to prevent sticking.
Add the cookie dough to the skillet, filling it 1/2 to 2/3 of the way to the top as the cookies will rise.
Pop in the oven to bake for about 12 to 14 minutes until the edges are golden and the center is still a bit soft. The pizookie will continue baking out of the oven.
Add vanilla ice cream scoops on top and hot fudge sauce if desired to enjoy!