Whisk the buttermilk and cream together, then pour into a plastic or glass jar.
Cover the top with a towel or cheesecloth, and use a rubber band to secure it.
Set the cream in a room temperature area and let ferment for 24 to 28 hours.
After thickening, place in the refrigerator for another 24 hours until use.
Store with an airtight lid for up to two weeks.
Notes
Avoid Ultra-pasteurized cream and buttermilk. The creme Fraiche will not be able to ferment. Pasteurized may work but will likely need more time to ferment.