Preheat your oven to 350°F.
Grease a cast iron skillet with butter and lightly coat it with flour. Set it aside.
Then whisk together the all-purpose flour, cocoa powder and salt in a separate bowl.
Melt the butter and chocolate chips together in either a double boiler or microwave safe bowl until silky and smooth.
Stir in the vanilla extract afterwards.
One at a time, add eggs into the mixture until thoroughly combined. Slowly sift in the dry ingredients until all incorporated.
Create an even layer of batter within your prepared skillet before baking for about 22-25 minutes or until a toothpick inserted through the center returns clean.
Let cool to room temperature before serving, with a scoop of vanilla ice cream if desired!