Heat oven to 425°F. Slice the top off garlic bulb, until the cloves are exposed and drizzle with olive oil. Wrap in aluminum foil. Bake until garlic is soft and golden, about 40 minutes. Remove from oven. When until cool enough to handle, squeeze the cloves out into a bowl. Set aside.
Place potatoes in a large pot. Add enough water to cover the potatoes and add about 2 tablespoons of salt. Bring to a boil, then lower the heat and simmer, uncovered, for about 10 to 12 minutes, until the potatoes fall apart easily when pierced with a fork.
As soon as the potatoes are tender, drain them in a colander.
Place the hot potatoes in a mixing bowl and using a paddle attachment, mix until smooth, about 1 minute.
Slowly add in garlic, miso, butter and milk and gradually turn up speed and mix for 1 minute longer.
Switch to whipping attachment and beat for about 1 minute, or until fluffy.
Season with salt and pepper, to taste.
Serve warm topped with extra butter (if desired) and chopped chives.