With a sharp knife, cut top 1/2 inch off garlic head. Set garlic on a sheet of foil and top with butter. Draw sides of foil up and pinch together to enclose garlic in a loose, sealed pouch.
Bake in a 400° oven until garlic is soft when pressed, 45 to 50 minutes. Unwrap and let stand until cool enough to handle.
Meanwhile, cut cauliflower heads into 1-inch florets, discarding leaves and stems. In a 4- to 5-quart pan, combine cauliflower and broth. Cover, bring to a simmer over high heat, then reduce heat and simmer until cauliflower is very tender when pierced, 12 to 15 minutes.
Squeeze garlic cloves from papery skins into pan with cauliflower and broth; pour in cream. Working in batches, transfer to a blender and whirl until smooth. Return soup to pan and add the nutmeg and salt and pepper to taste.
Set pan over medium-low heat and stir occasionally until hot. Pour into a large bowl or tureen and sprinkle with chives.