Let 1 quart of heavy cream sit overnight to ripen in a large earthenware bowl. The bowl should be no more than half full. In the morning beat at low speed with an electric mixer until the cream breaks into grain sized lumps of solid butter and liquid buttermilk.
Pour off the buttermilk and rinse the butter thoroughly with cold water until the rinse water is clear. Drain butter on a cheesecloth in a colander in the kitchen sink.
Transfer the drained butter to a damp wooden bowl or board. Pat it flat with the back of a wooden spoon or a pair of wet butter paddles. Pour off any remaining water. Pile butter up and flatten again repeatedly until all excess water has been squeezed out and the butter has an even spreadable consistency.
You may add a little salt now if you desire, mixing thoroughly with the spoon or paddles.
Shape into pats, or place in a mold, wrap in wax paper and refrigerate. Yields app. 1 pound.