We found that ripening the cream a bit first—in essence, culturing it—yields a more fully flavored butter. Start by pouring a quart of the best-quality organic heavy cream into a large bowl; then cover the bowl with plastic wrap and let the cream sit at room temperature (ideally, around 68 to 70 degrees Fahrenheit), undisturbed, for six hours so that it begins to sour slightly. In the meantime, line a colander with cheesecloth and set it inside another bowl.