How to Make It
Culture the Cream. Heat cream in a saucepan over low heat until a thermometer registers 70°. Pour cream into a large bowl; stir in store-bought buttermilk. Cover tightly with plastic wrap, and let stand at room temperature, free from drafts, 24 hours. (Place the bowl in an oven that's not in use, and close the door.) Mixture will thicken and resemble pancake batter. You'll get the best results if you use cream that is not ultra-pasteurized. Strain and Drain. Process thickened mixture in a food processor 5 to 8 minutes or until small bits of butter begin to form. Pour through a fine wire-mesh strainer into a large bowl. Let strainer stand over bowl 10 minutes, occasionally pressing butter with a rubber spatula to drain buttermilk thoroughly. Use buttermilk immediately, or pour into a glass jar, cover tightly, and chill up to 2 weeks. Run cold water over butter in strainer; squeeze to form a ball. Rinse, Shape, Chill. Knead butter in ice water until water becomes cloudy. Drain; repeat process until water remains clear. Knead in kosher salt. Shape into a disk, wrap, and chill up to 3 weeks.