Lightly grease an 8 x 12 inch baking tray with cooking oil spray. Line with parchment paper (or baking paper) and set aside.
Combine melted butter, oil and sugars together in a medium-sized bowl. Whisk well to combine. Add the eggs and vanilla; beat until lighter in colour (another minute).
Sift in flour, cocoa powder and salt. Gently fold the dry ingredients into the wet ingredients until JUST combined (do NOT over beat as doing so well affect the texture of your brownies).
Fold in 3/4 of the chocolate pieces.
Pour batter into prepared pan, smoothing the top out evenly, and top with remaining chocolate pieces.
Bake for 25-30 minutes for just under-done brownies(fudgier texture) or until the centre of the brownies no longer jiggles and is JUST set to the touch. OR if you prefer well set and firmer brownies, keep in the oven for 35-40 minutes.
NOTE: Brownies will continue baking once they are taken out of the oven. If testing with a toothpick, the toothpick should come out dirty for fudge-textured brownies.
After 15-20 minutes, carefully remove them from the pan and allow to cool at room temperature before slicing into 16 brownies. They set while they cool.