Preheat oven to 350° F. Place the garlic on a small sheet of foil and pull up the foil's edges to form a pouch. Pour the oil over the garlic and sprinkle with about ¼ teaspoon of the salt. Roast for 35 minutes. Allow to cool slightly. Place the butter in a small bowl. Squeeze the garlic out of its skins onto the butter. Add the remaining salt and mash with a wooden spoon to combine. Chill the butter until needed, but allow it to come to room temperature before spreading.