Find a large jar with a tight-fitting lid that will hold the cream about half full. Refrigerate the jar for at least 1 hour. Pour the cream into the cold jar. Tightly secure the lid and shake as hard as possible until chunks of butter start to form, 15 to 30 minutes. Pour into a strainer set over a bowl. The chunks in the strainer are butter, and the liquid in the bowl is buttermilk.

Pour the buttermilk into a clean container, cover, refrigerate, and reserve for another use. Turn the butter into a clean bowl and cover with very cold water. Pour into a strainer, discarding the liquid. Continue rinsing the butter with very cold water until the water runs clear. (The cloudy water is buttermilk which will make the butter turn sour.) When the butter is clean, work with a wooden spoon or rubber spatula to press out any remaining liquid. Discard this liquid. If desired, add salt to the butter. (Salted butter will keep longer.)

Transfer the butter to a clean container for keeping, pressing with a wooden spoon or spatula to dispel any air bubbles. Refrigerate until ready to use.