Preheat the oven to 350°F. Grease and line a 12×8 baking pan with parchment paper.
In a large bowl, combine melted butter, oil, granulated and brown sugar. Whisk until smooth.
Add vanilla and eggs, beating for 30 seconds until the batter lightens.
Sift in flour, cocoa powder, and salt. Mix lightly until just combined.
Pour the batter into the prepared pan and spread evenly. Sprinkle the remaining chocolate chunks on top.
Bake for 25-30 minutes until the center no longer jiggles. For fudgy brownies, avoid overbaking. For cakey brownies, bake 5-8 minutes longer.
Remove from the oven and let cool completely.