Whipped ganache is best for filling; unwhipped makes a glossy frosting.
Put chocolate in a dry metal bowl. In a small saucepan, scald cream; pour immediately over chocolate. Let stand for a few minutes, then stir gently until smooth.
Pour half the ganache into a bowl and refrigerate until cold but not solid. Whip until lighter in color and stiff enough to spread; use as soon as possible. Keep remainder over a bowl of warm water until ready to use.