Slice ¼ inch off the top of the garlic bulb. Place the bulb, cut side up, on a 12x12 inch piece of aluminum foil, folded in half. Pour the olive oil over the top of the cut garlic bulb. Pull the edges of the foil around the garlic bulb to enclose the garlic in foil.
Place in the oven and roast for 45–60 minutes, or until the garlic cloves are deep brown.
Squeeze the roasted garlic out of the skin and mash with a fork.
Meanwhile, cut the potatoes (with the skin on) into 1½ inch chunks. Place in a large saucepan and cover with water. Add 1 teaspoon of salt to the water.
Place over high heat and bring to a boil. Reduce to a simmer and cook for 15–20 minutes, or until a fork easily slides in and out of the middle of a potato.
Drain the potatoes. Place them back in the large saucepan or another mixing bowl.
Add the mashed roasted garlic, butter, buttermilk, and 1½ teaspoons of salt.
Using a handheld mixer, mix on medium speed until mashed, about 3–4 minutes. Taste and season with additional salt if desired. Serve immediately.
Notes
Yukon Gold potatoes can also be mashed with the skin on. Make these dairy-free by substituting non-dairy butter for the butter, and non-dairy milk (such as unsweetened almond) for the cow's milk. These can be made up to 2 days in advance. Store in the refrigerator until you're ready to serve. To reheat, place in a covered dish in the microwave and heat until hot throughout. Add additional buttermilk to loosen the potatoes as necessary.