Preheat the oven to 400 degrees. Slice the top 1/3 off from the whole garlic bulb. Place it in the center of a sheet of aluminum foil. Drizzle all over the top with olive oil. Gather the foil up at the top and close it tightly. Place it directly on the center rack of the oven. Roast for 45-55 minutes. The cloves will be golden brown and completely fork tender. Allow to cool before handling.Quarter the potatoes and place them in a large pot will a tablespoon of salt and fill the pot with water. Bring the water to a boil, then turn it to a simmer. Continue to simmer for about 30 minutes until they are completely fork tender. While the potatoes are boiling, remove the roasted garlic cloves from from the bulb and place them in a mixing bowl with the room temperature butter. Use a fork to mash the garlic into the butter. Drain the boiled potatoes and use a food mill or potato masher or any other method you prefer to mash them to your desired consistency. Mix the potatoes with the roasted garlic butter, then stir in 1/2 cup of buttermilk. Serve hot. If the mashed potatoes are not served immediately and lose their heat, pour them back into the pot you cooked them in along with 1/4 cup more of buttermilk and heat on low with the lid on until serving.