Peel and cut the potatoes: Peel and cut into even chunks, about 1.5-inch pieces. Even sizes cook evenly, which means smoother mash later.
Start in cold water: Place potatoes in a large pot and cover with cold water by about an inch.
Add 1 tablespoon kosher salt and the smashed garlic if using.
Bring to a gentle boil: Set over medium-high heat and bring to a boil, then reduce to a simmer. Cook 15–20 minutes, until a knife slides through without resistance.
Drain well: Drain the potatoes thoroughly and remove the garlic. Return them to the hot pot.
Let them sit 1–2 minutes on low heat to steam off excess moisture.
Warm the dairy: In a small saucepan, gently warm the milk (or half-and-half) with the butter until the butter melts. Don’t boil. Warm dairy blends in smoothly.
Mash while hot: Use a potato ricer for the fluffiest texture, or a hand masher for a slightly rustic finish.
Avoid food processors or blenders; they make potatoes gluey.
Add butter first: Pour in the melted butter mixture gradually, stirring gently with a spatula. Start with about half, then add more until it looks creamy but not soupy.
Adjust with sour cream: Stir in sour cream or crème fraîche if using. This adds body and a slight tang that brightens the richness.