For the best results from this mashed potato recipe, I recommend heating the butter and milk. I'll admit that I don’t always do this step myself if I'm feeling lazy or impatient or when I'm making Shepherd's Pie; however, it really is best to heat them up first. Doing so helps the potatoes absorb the liquid better (rather than shocking them with cold milk), and it reduces the risk of over-mixing because things will combine easier. Now, if you find yourself needing to add just a splash more milk or wanting another pat of butter, you can just toss it in--no need to warm it first.