Place potatoes and garlic cloves in a large pot and cover with cold water by 1 inch. Add 1 tablespoon salt and bring to a boil over high heat.
Reduce heat to medium and simmer for 15-18 minutes, until potatoes are fork-tender and easily pierced.
Drain potatoes and garlic well, then return them to the hot pot for 1 minute to let excess moisture evaporate.
Add butter to the pot and mash potatoes and garlic together using a potato masher until mostly smooth.
Stir in warm milk and sour cream until creamy and well combined. Season with pepper and additional salt to taste.
Transfer to a serving bowl and garnish with fresh chives if desired. Serve hot!