Peel your potatoes and cut them into roughly one-inch cubes so they cook at the same rate. I always rinse the cubes under cold water first to wash away excess starch, which keeps the final mash from turning gummy. Try to keep the pieces uniform in size, since uneven chunks mean some potatoes turn mushy while others stay firm.
Place the potato cubes in a large pot and cover them with about an inch of cold water, then stir in the salt. Bring everything to a gentle boil over medium-high heat and let the potatoes cook for 15 to 20 minutes. You'll know they're ready when a fork slides through a piece with almost no resistance.
Drain the potatoes in a colander right away so they don't sit in hot water and turn waterlogged. I like to return them to the warm, empty pot for a minute or two over low heat, which lets any leftover moisture steam off. This tiny step makes a real difference in how creamy the final texture turns out.
While the potatoes are draining, gently heat the milk and butter together in a small saucepan until the butter melts completely. Warming the dairy first is one of my favorite tricks, because cold milk poured straight into hot potatoes can make them seize up and turn starchy instead of silky.
Using a potato masher or ricer, mash the potatoes directly in the pot until no large lumps remain. A ricer gives the smoothest, most restaurant-quality texture, but a sturdy masher works great too if that's what you have on hand in your kitchen.
Slowly pour in the warm milk-and-butter mixture while gently folding the potatoes with a spoon or spatula. Add the sour cream here if you're using it, along with the black pepper. Stir just until everything is combined and creamy, being careful not to overwork the potatoes.
Give your mash a taste and add more salt or pepper as needed, since potatoes can vary in how much seasoning they soak up. I always adjust at this final stage rather than guessing earlier, because it's much easier to add more than to fix an over-salted batch.
Transfer the mashed potatoes to a serving bowl and top with a pat of butter and a sprinkle of fresh chives or parsley. Serve them immediately while they're warm, creamy, and at their absolute best for your holiday table.