Preheat oven to 350 F.
Lightly grease an 8-inch square baking pan with cooking oil spray. Line with parchment paper; set aside.
Combine melted butter, oil and sugar together in a medium-sized bowl. Whisk well for about a minute. Add the eggs and vanilla; beat until lighter in color.
Sift in flour, cocoa powder and salt. Gently fold the dry ingredients into the wet ingredients until just combined.
Pour batter into prepared pan, smoothing the top out evenly.
Bake for 20-25 minutes, or until the centre of the brownies in the pan no longer jiggles and is just set to the touch.