Preheat oven to 350 degrees and lightly grease a 6-8" cast iron skillet.
In a medium microwave-safe bowl, melt the butter on high in 15 second intervals.
Add in the sugars and mix with a spoon until combined.
Add in egg and vanilla, stirring to incorporate.
Add flour, salt, baking soda and mix with a spoon until completely combined.
Stir in chocolate and spread evenly into prepared skillet (feel free to throw some extra chocolate chunks on top!).
Bake at 350 degrees F for 20-22 minutes or until edges are crispy and center is just set (if using a 6 inch pan, you may need up to 25 minutes bake time, if you're using individual skillets or ramekins, you may only need 12-15).
Let cool slightly before topping with ice cream and serving.