In a small saucepan, heat the cream just long enough to take the chill off—to bring it to body temperature. If you have ready access to a low-range instant-read thermometer, it should be about 100 degrees. Off the heat, whisk in the sour cream and pour into a glass jar. Set the lid on the jar (but don’t tighten it), then place the jar in a warmish place (it shouldn’t be over 90 degrees). After 12 hours, the cream should be noticeably thicker. Refrigerate (you can tighten the lid now) for at least 4 hours to complete the thickening. Crema will last for at least a week in the refrigerator.