Place the dry ingredients (all purpose flour, baking soda, baking powder, corn starch, salt and chocolate chips) into a large mixing bowl and combine. Set this aside.
Now in a separate bowl, combine the sourdough discard or sourdough starter, melted butter, brown sugar, white sugar, egg + egg yolk and vanilla extract. Stir together until they resemble runny caramel.
Now pour the wet ingredients into the dry ingredients and stir to form the cookie dough.
At this point, you can ferment the dough for up to 24 hours, but this is not essential. You can skip this if you want to make your sourdough cookie skillet more quickly.
If you want to ferment your sourdough skillet cookie dough, cover the bowl with plastic wrap and place into the fridge for 12 to 24 hours.
When you are ready to bake the sourdough cookie dough, preheat the oven to 175C (350F) lightly grease a cast iron skillet.
Press the cookie dough into an even layer in your cast iron skillet. I don't grease my cast iron skillet (however I always add a layer of olive oil when I clean my skillets so they are already "pre greased and seasoned".If you don't have a cast iron skillet, you can bake this in any oven safe skillet or fry pan or even a pie dish or pie plate. Bake the sourdough cookie skillet for around 25 to 30 minutes at 175C or 350F. I like to remove the sourdough cookie skillet from the oven when the middle is still a bit soft and gooey.
Once the cookie skillet is cooked to your liking, remove from the oven to cool a little. I like to remove the sourdough cookie skillet from the oven when the middle is still a bit soft and gooey.