Place the ingredients in a clean glass container with a cover, and let them sit in a warm area for 24 hours, until it thickens (the bacteria in the yogurt ferments the cream and causes it to thicken).
Uncover and stir. If it’s not thick, let stand for a few more hours. When it’s ready, place the mixture in the refrigerator, where it will continue to thicken. It stores for up to 2 weeks.