In a jar with a lid, place the cream and buttermilk; cover securely and shake for 15 seconds. Remove the lid and cover with the cheesecloth, secured with a rubber band or string. Set aside at room temperature for 12 to 24 hours, until thickened similar to sour cream. (This time frame varies depending on the cream and the temperature of your home.)
Stir the thickened crème fraîche well. Replace the lid tightly, and refrigerate for at least 6 hours before serving; it will become even thicker as it chills. Store in the refrigerator for up to 2 weeks.
Notes: Although it is generally recommended to avoid the use of ultra-pasteurized dairy products for culturing, I have found through many test batches that ultra-pasteurized cream works as well as all other types of cream in making crème fraîche.If using a packaged crème fraîche starter, follow the instructions provided with the starter.