Preheat oven 400°F. Slice garlic heads tops off. Place on foil square. Drizzle 1 tsp oil each, wrap tight. Roast baking sheet 40 minutes till soft squeezeable. Cool, squeeze cloves into bowl.
Fill large pot cold water. Add potato chunks and 1 tbsp salt from measuring spoon. Bring boil high heat. Simmer 15-20 minutes till fork tender. Drain colander, return pot dry heat 1 minute steam off.
Mash potatoes coarse using potato masher. Add roasted garlic, softened butter cubes from measuring cup. Pour warm milk gradual with measuring cup. Mash vigorous till fluffy smooth no lumps.
Stir salt, pepper from measuring spoons. Whip electric mixer or spoon 1 minute peaks form. Fold chives if using with spoon. Taste adjust salt.