Melted butter is best for homemade brownies when you want them to be nice and fluffy with rich flavor. The butter helps the recipe rise while adding depth to the flavor. In contrast, the oil, like vegetable oil, will not aid in leavening and therefore creates more dense brownies without the hint of buttery taste. Either will work to make brownies from scratch, but butter yields the best results. However, when it comes to boxed brownie mix, you should follow the instructions on the box.