Preheat oven to 400°F (200°C). Slice the top off the garlic head to expose cloves, drizzle with olive oil, wrap in foil, and roast for 35-40 minutes until soft.
While garlic roasts, peel and cube the potatoes. Add them to a pot of salted water and bring to a boil. Cook for 15-20 minutes or until fork-tender.
Drain potatoes and return to the pot. Squeeze roasted garlic cloves into the pot and add butter.
Mash until smooth, then stir in sour cream and milk. Adjust consistency with more milk if desired.
Season with salt and pepper to taste. Garnish with chopped chives before serving if desired.