Roast the garlic. Heat the oven to 400 degrees F. Slice the top 1/4 inch off each head of garlic, drizzle with the olive oil, wrap in foil and roast 40 to 45 minutes, until soft and golden. Cool slightly and squeeze out the cloves.

Cook the potatoes. Place the potatoes in a large pot and cover with cold water by about an inch. Salt the water generously. Bring to a boil, then reduce to a steady simmer and cook 15 to 20 minutes, until a fork slides through easily.

Make the rosemary brown butter. While the potatoes cook, melt the butter with the rosemary sprigs in a light-colored saucepan over medium heat, swirling as it foams. Cook 5 to 8 minutes, until the milk solids are golden brown and smell nutty. Pour into a heatproof bowl, scraping in the browned bits, discard the sprigs and reserve 2 tablespoons for finishing.

Warm the dairy. Warm the cream, milk and roasted garlic cloves in a small saucepan over low heat for 5 minutes without boiling. Mash the garlic into the cream with a fork.

Drain and dry. Drain the potatoes well. Return them to the hot pot over low heat for 1 to 2 minutes, shaking gently, until the excess moisture evaporates.

Rice. Press the hot potatoes through a potato ricer for an extra-fluffy mash. Do not use a blender or food processor.

Enrich and season. Fold in the brown butter, except the reserved 2 tablespoons, along with the chopped rosemary. Then add the warm garlic cream, starting with about 1 cup and adding more as needed. Season with salt and pepper.

Serve. Transfer to a warm bowl, drizzle with the reserved brown butter and sprinkle with chives, if using.