Combine the melted butter, oil, and sugars together in a bowl and beat until smooth.
Add the eggs and vanilla and beat until the batter turns a lighter color.
Sift in the dry ingredients and stir to combine. Do NOT overmix the batter. Only mix until everything is homogenous.
Fold 3/4 of the semisweet chocolate pieces into the batter.
Pour the batter into a parchment paper lined 9 x 13 inch pan.
Top the brownies with the reserved chocolate.
Bake for 25 minutes for fudgy brownies or 30-35 minutes for firmer brownies. Fudgy brownies are done when the top is set to the touch and the center isn’t jiggly anymore. Firmer brownies are ready when a toothpick comes out clean. (Note: The toothpick won’t come out clean for fudgy brownies.)
Remove from heat and allow to cool in the pan for 15-20 minutes. Then carefully take them out of the pan (this part is easier with a second pair of hands) and set on a wire rack to cool fully before serving.