Homemade Creme-Fraiche

To make 1 cup of crème fraiche, pour 1 cup heavy (whipping) cream into a clean jar, add 2 tablespoons buttermilk or yogurt, cover the jar tightly, and shake it for about a minute. Then just leave the jar at room temperature for 12 to 24 hours, or until the crème fraiche thickens slightly. How quickly it thickens will depend on the temperature of the room - the warmer the room, the faster it will thicken.
When it is nice and thick, chill the crème fraiche in the refrigerator for a day before you use it.
It can be kept in the refrigerator, covered, for about 2 weeks and will get a bit tangier day by day.