Peel, rinse and chop the potatoes into small 1-inch cubes or chunks. Fill a large saucepan with cold water and season with a pinch with kosher salt.
Place the potatoes in the pot and bring to a boil over medium-high heat. Reduce to medium-low heat and simmer for 20 minutes.
Stir potatoes occasionally and pierce with a fork to test they are done. Drain the potatoes in a colander then return to the pot.
Heat the milk in a small saucepan or in the microwave in a microwave-safe bowl. Next, add the hot milk before mashing the potatoes with a masher.
Transfer the potatoes to an airtight container or metal bowl covered with plastic wrap or aluminum foil and refrigerate for up to 2 days.
Before serving, return the cold mashed potatoes to a pot over medium heat or slow cooker on low and add the melted butter.
Stir the potatoes occasionally to fluff and incorporate the butter. Keep warm until serving time.
Transfer the warm potatoes to glass bowl or serving dish and serve. Season with salt and black pepper.