You may use an actual cream culture from a cheesemaking supply company to inoculate the cream, but a number of commonly available products have active cultures in them. To every 1 quart heavy cream, add ½ cup plain yogurt, kefir, crème fraîche, or cultured buttermilk. To use a cream culture, add as much as directed on the package. Let the cream sit at room temperature in a covered jar or bowl for 24 hours, chill in the refrigerator until it is around 50°F, and then proceed as for Homemade Sweet Cream Butter.