In a medium saucepan set over medium heat, stir the butter, oil, chocolate, and sugar until melted and smooth, about 5 minutes. (Or place in a large microwave-safe bowl and microwave in 20-second increments, stirring after each increment, until melted and smooth.) Pour into a large mixing bowl and whisk to make sure it’s completely smooth and not grainy. Set aside to slightly cool for 5 minutes.

Adjust the oven rack to the lower third position and preheat oven to 350°F (177°C). Lightly grease a 10-inch cast iron skillet.

Add the eggs and vanilla to the bowl with the melted chocolate, and whisk until combined. Stir in the cocoa powder, flour, and salt. Once combined, fold in 1 cup chocolate chips.

Pour batter into the prepared skillet and sprinkle the remaining 2 Tablespoons chocolate chips over the top. Bake for 30–36 minutes, or until a toothpick inserted in the center comes out with a few moist crumbs. Err on the side of underbaking, which gives you that gooey brownie center.

Allow the skillet brownie to cool for 5 minutes before serving.