Preheat the oven to 325F then spray a 10-inch cast iron skillet with non-stick spray and set aside.
In a microwave-safe bowl melt the butter, oil, both sugars, and cocoa powder together in 30-second intervals mixing in between every time. Once fully melted microwave for another 1 minute and stir vigorously to allow the sugar to melt fully.
⅔ cup butter, ⅓ cup vegetable oil, 1 cup granulated sugar, ½ cup brown sugar, ¾ cup cocoa powder
Whisk in the eggs one at a time then add in the vanilla. You want the batter to pull from the bowl a little bit.
2 egg, 1 teaspoon vanilla extract
Add in the flour, and the salt and fold just until combined. Fold in the chocolate chunks. Sprinkle some extra chocolate on top if you want.
½ teaspoon salt, ⅔ cup all purpose flour, 8 oz semi-sweet chocolate bar
Pour the batter into the skillet and bake for30 minutes.
Allow the brownies to cool down for 10 minutes on a wire rack before topping with some ice cream and digging in.