Recipe Source: Baking: From My Home to Yours by Dorie Greenspan, page 459
1 cup heavy cream
1 tablespoon buttermilk or plain yogurt (I used the plain yogurt)
Directions: Put the cream and buttermilk or yogurt in a clean jar with a tight-fitting lid. Cover the jar and shake for a minute or so.
Put the jar on a counter and leave it for 12 to 24 hours, or until the cream thickens slightly. How long this will take depends on how warm your room is - the warmer the room, the quicker the thickening. (Mine took about 22 hours. At 21 hours, it was still liquid, and then at 22, bam it was thick.)
When the cream is thick, transfer the jar to the refrigerator and let it chill for 1 day before you dip into it.