Preheat oven to 400 degrees. Slice 1/4 inch off top end of garlic bulb to expose tops of cloves. Wrap the garlic in a piece of aluminum foil, cut side up and spoon oil over cut side. Wrap foil to seal tightly.
Bake 45 minutes, or until cloves are tender when pierced with a sharp knife. Let stand 5 minutes. Squeeze garliccloves from skins into small bowl. Smash with back of spoon and set aside.
Cook potatoes in pot of boiling salted water 12 to 15 minutes, or until tender. Drain and transfer to a mixing bowl. Combine sour cream and milk in a small microwaveable bowl. Microwave on high for 1 minute, or until warm. Add sour cream mixture, garlic, and 3 1/2 tbsp of the chives to the potatoes. Beat with an electric mixer until fluffy. Season with salt and pepper.