Preheat the oven to 180℃, brush your 4/5 inch skillet cookie with a little oil, set aside.
In a small mixing bowl add the butter, brown sugar and beat with a rubber spatula until pale and creamy, around 2 minutes.
Add the milk, vanilla and mix until evenly combined.
Add the flour, cocoa powder, corn flour, baking powder, baking soda, salt, chocolate chips and and fold together until completely mixed through.
Transfer half the dough to the skillet and spread it out evenly. Dollop the peanut butter in the center and spread it out a little, leaving a small border around the edges. Add the remaining cookie dough on top and smooth it out with your spatula.
Top it with a few chocolate chunks and bake for 15-18 minutes until the top is set but the center is still soft and gooey.