Expose the cloves of garlic head by slicing off the top 1/2 inch.
Place them in a small baking dish, and drizzle with olive oil.
Cover tightly with aluminum foil and bake about 55 minutes, or until very tender when pierced with a small knife.
Remove from oven, uncover, and allow to cool.
Break apart the garlic heads.
Squeeze each garlic clove into a small bowl and mash with a fork until almost smooth. You should end up with about 1/3 cup. Reserve any olive oil remaining in the baking dish.
Pour 1-inch of water into a large pot and bring to a boil.
Place the potatoes into a collapsible steamer basket and set the basket over the boiling water. (Do not allow the water to touch the bottom of the steamer basket). Cover and steam about 15 minutes, or until the potatoes are tender when pierced with a small knife. Transfer potatoes to a bowl.
Empty the pot and wipe dry.
Return the potatoes to the warm pot.
Add the butter and 4 tablespoons of the roasted garlic and mash well with a potato masher.
Add the 6 tablespoons milk, 1 teaspoon kosher salt, and 1/2 teaspoon pepper.
Continue to mash, adding more milk, 1 tablespoon at a time, in order to reach the desired consistency.
Taste and add more of the roasted garlic, if desired.
Serve at once, drizzled with the reserved garlic oil.