Good plain bread spread with good plain butter is one of the simplest pleasures. We owe butter’s buttery consistency to the peculiar melting behavior of milk fat, which softens and becomes spreadable around 60°F/15°C, but doesn’t begin to melt until 85°F/30°C.
This workable consistency also means that it’s easy to incorporate other ingredients into the butter, which then carries their flavor and color and helps apply them evenly to other foods. Composed butters are masses of room-temperature butter into which some flavoring and/or coloring has been kneaded; these can include herbs, spices, stock, a wine reduction, cheese, and pounded seafood. The mixture can then be spread on another food, or refrigerated, sliced, and melted into a butter sauce when put onto a hot meat or vegetable. And whipped butter prepared by the cook is butter lightened by the incorporation of some air, and flavored with about half its volume of stock, a puree, or some other liquid, which becomes dispersed into the butter fat in small droplets.