Step 1: Preheat your oven to 350°F (175°C). Lightly grease a 9-inch cast iron skillet or other oven-safe skillet.
Step 2: In a large bowl, combine the melted butter and vegetable oil. Whisk in the granulated sugar until well combined.
Step 3: Add the eggs one at a time, whisking well after each addition until the mixture is smooth and glossy. Stir in the vanilla extract.
Step 4: In a separate medium bowl, whisk together the all-purpose flour, salt, baking powder, cocoa powder, and dark cocoa powder until thoroughly combined.
Step 5: Gradually add the dry ingredients to the wet ingredients, mixing with a spatula or spoon until just combined. Be careful not to overmix. Fold in the dark chocolate chips or chunks.
Step 6: Pour the brownie batter into the prepared skillet, spreading evenly.
Step 7: Bake for 25-30 minutes, or until the edges are set and a toothpick inserted into the center comes out with moist crumbs (not wet batter). The center should still be slightly fudgy. Do not overbake!
Step 8: Remove from the oven and let cool slightly before serving. For an extra decadent treat, serve warm with a scoop of vanilla ice cream.