Step 1: Peel the potatoes and cut them into roughly 1-inch uniform cubes. Rinse the cut potatoes under cold water to remove excess starch. Place them in a large pot.
Step 2: Cover the potatoes with cold water by about an inch. Add a generous amount of salt to the water. Bring to a boil over high heat, then reduce the heat to medium-low and simmer until the potatoes are fork-tender, about 15-20 minutes.
Step 3: Once tender, drain the potatoes thoroughly in a colander. Let them sit in the colander for a minute or two to allow steam to escape. Return the potatoes to the empty, hot pot (off the heat).
Step 4: Add the unsalted butter to the hot potatoes in the pot. Let it melt for a minute, then begin mashing the potatoes using a potato ricer, masher, or sturdy fork until mostly smooth.
Step 5: Gradually add the warm milk (or half-and-half) to the mashed potatoes, stirring gently with a spatula or wooden spoon until combined and the desired creaminess is achieved. Start with half the amount and add more as needed.
Step 6: Season the mashed potatoes generously with salt and freshly ground black pepper. Taste and adjust seasoning as necessary. Add an optional pinch of nutmeg if desired.
Step 7: Serve The Best Mashed Potatoes Recipe immediately. Garnish with fresh chives if desired.