Remove the lid. Mash the cooked potatoes in a medium mixing bowl with a potato masher. Don’t mash the potatoes too aggressively, as that will make the mashed potatoes gluey and unappetizing. So be careful if you’re using a food processor to mash the potatoes. Add half of the garlic butter mixture to the bowl. Continue to mash, stir, and add the mixture until desired consistency. Add Parmesan cheese.