Butter is one of the most flavorful and versatile of all culinary fats, but anyone who's ever tried to spread hard butter on soft bread knows that it has shortcomings as well. Butter must be refrigerated to remain fresh-tasting for more than a few days, but once refrigerated, it's too firm to spread easily. One way to counter that difficulty is to whip air into the butter, which lightens and softens its texture.
The simplest form of whipped butter contains no other ingredients; instead, it's simply butter whipped to incorporate air in much the same way as whipped cream. It will hold its shape at cool room temperatures and remain fresh for just as long as regular butter, but it's also soft enough to spread -- especially on hot foods -- right out of the refrigerator. Frugal cooks can "stretch" the butter and make it more spreadable by adding small quantities of water, milk, cream, buttermilk or oil to the butter as it whips. Water and oil have little effect on its shelf life, but whipped butter containing milk, cream or buttermilk is more perishable and must be kept refrigerated.