Preheat your oven to 350 degrees F. and grease a 10-inch cast iron skillet.
Add 2 cups of chocolate chips and the butter to a bowl, then microwave in 20-second intervals, stirring each time, until fully melted.
Once the chocolate is smooth, stir in the brown sugar, granulated sugar, unsweetened cocoa powder, salt, and vanilla extract until well combined.
Add the eggs one at a time and mix until well incorporated.
Fold in the flour and mix until well incorporated and then fold in 1 cup of chocolate chips.
Pour the batter into the skillet and spread it evenly. Bake for 25–35 minutes or until a knife inserted in the center comes out mostly clean. A few moist crumbs are fine, just make sure not to overbake.
Serve warm with vanilla ice cream and enjoy!