Peel potatoes and cut into 1 1/2-inch chunks.
Place in a large pot, cover with cold water by 1 inch.
Bring to a boil, then simmer until fork-tender, 15-20 minutes.
Warm milk and melt butter together until steamy, not boiling.
Drain potatoes well; return to the hot pot over low heat 1 minute.
Mash potatoes. Gradually pour in warm milk-butter mixture.
Stir in sour cream, salt, pepper, and garlic powder until creamy.
Taste and adjust seasoning. Garnish with chives and serve hot.