Preheat your oven to 350°F (175°C). Grease and line an 8x8-inch baking pan with parchment paper, leaving a slight overhang for easy removal.
In a large mixing bowl, whisk together the melted butter, oil, sugar, and brown sugar until smooth and glossy.
Add the eggs and vanilla extract. Beat until fully combined and slightly thickened.
Sift in the flour, cocoa powder, and salt. Gently fold with a spatula until just combined. The batter should be thick but not dry. Avoid overmixing.
Stir in chocolate chips and/or nuts if using.
Spread the batter evenly into the prepared pan, smoothing the top.
Bake for 28–32 minutes, or until the edges are set and a toothpick inserted in the center comes out with a few moist crumbs (not wet batter).
Allow the brownies to cool in the pan for at least 20 minutes before lifting them out with the parchment and cutting into squares.