Personally, I like to use a hand mixer* (affiliate link) to make the mashed potatoes super smooth. The potatoes should be soft enough that they can very easily blend with the other ingredients. When the potatoes are fork tender, drain, then toss them back in the saucepan (or Dutch oven) with the butter, milk and parmesan. Blend slowly with the hand mixer and pick up speed as the ingredients meld together. Blend until whipped and silky smooth.