Preheat the Oven: Set the oven to 175°C (350°F). Lightly grease an 8x12-inch baking pan with cooking oil spray and line it with parchment paper. Set aside.
Mix Wet Ingredients: In a medium-sized bowl, combine the melted butter, vegetable oil, and both sugars. Whisk well to combine. Add the eggs and vanilla extract, then beat until the mixture is lighter in color (about 1 minute).
Add Dry Ingredients: Sift in the flour, cocoa powder, and salt. Gently fold the dry ingredients into the wet ingredients until just combined. Tip: Do not overmix, as this will affect the texture of your brownies.
Add Chocolate: Fold in ¾ of the chocolate pieces, then pour the batter into the prepared pan. Smooth the top evenly and sprinkle the remaining chocolate pieces on top.
Bake:
For fudgier brownies, bake for 25-30 minutes, or until the center no longer jiggles and is just set to the touch.
For firmer brownies, bake for 35-40 minutes.
Note: The brownies will continue to bake and set in the pan once out of the oven. If testing with a toothpick, it should come out dirty for a fudgier texture.
Cool and Slice: Let the brownies cool in the pan for 15-20 minutes before carefully removing them. Allow them to cool completely to room temperature before slicing into 16 pieces.
Enjoy!