Preheat the oven to 350 F.
Combine butter and sugars.
Add vanilla, egg, and yolk and mix well.
Stir in flour, cornstarch, and salt until well combined.
Stir in chocolate chips.
Press the dough into an 8" cast iron skillet with a spatula.
Bake at 350 F for approximately 30 minutes till the center is set.