Preheat the oven to 325 Fahrenheit (NOT 350!)
Grease a 6 inch cast iron skillet and set aside. In a small mixing bowl, add the melted butter and both sugars.
Beat on low speed for 1 minute.
Beat in the egg yolk and vanilla. Don't worry if you get a bit of egg white in there.
Add in the flour, baking powder and salt, making sure to spoon and level the flour, don't scoop from the bag.
Beat together until combined. The dough will be thick.
Fold in the chocolate chips until well combined.
Add to your prepared skillet and press down. Sprinkle on a few extra chocolate chips on top.
Bake for 20-25 minutes.
It will continue to cook quite a bit as it comes out of the oven because it will take time for the skillet to cool, so be sure to take it out early when it still looks gooey to prevent a dry texture.
Serve with vanilla ice cream and enjoy!