Put the potatoes in a large pot and cover with cold water by an inch. Add 1 tablespoon salt. Bring to a boil over high heat, then reduce to a steady simmer.
Cook until a knife slides through the chunks with no resistance, 15–20 minutes.
Meanwhile, combine the milk, butter and smashed garlic in a small saucepan. Warm over low heat for 10–15 minutes without boiling, until the garlic is soft. Remove the garlic, mash it with a fork and return it to the pan, or discard it for a milder flavor.
Drain the potatoes well and return them to the hot pot. Set over low heat for 1–2 minutes, shaking the pot, to steam off excess moisture.
Mash with a potato masher or pass through a ricer. Pour in the warm garlic milk in two or three additions, stirring gently with a spatula until smooth and creamy. Fold in the sour cream if using.
Season with salt and pepper, top with chives and serve hot.