Cover the potatoes and garlic with salted water in a large saucepan and bring to a boil. Simmer until tender, about 15 minutes. Drain; return the potatoes and garlic to the hot saucepan.
Meanwhile, in a small saucepan, bring the milk and cream to a simmer. Add 2 tablespoons of the butter. Pour the hot milk mixture over the potatoes and garlic. Season with 2 teaspoons salt and mash with a potato masher until smooth. Transfer the mashed potatoes to a 2-quart ovenproof baking dish. Cool to room temperature, then press the surface with a piece of plastic wrap. Refrigerate, covered, until ready to use.
Heat oven to 425° F. Let the potatoes stand at room temperature for 30 minutes. Remove plastic wrap and bake, covered, until warmed through, about 30 minutes.
Meanwhile, melt the remaining 6 tablespoons of butter. Combine with the scallions, chives, and ¼ teaspoon each salt and pepper. Drizzle the herb butter over the potatoes.