Combine cream with buttermilk or starter. Put into a clean, warmed thermos or glass jar. Cover tightly and keep in a warm place for 6 to 8 hours. If using creme fraiche starter, keep covered for 24 hours. When mixture has jelled and is almost firm, refrigerate. It will solidify further in the fridge. Will keep for a month if made with the Solait and for 10 days with the buttermilk.
NOTES : This is an old one from the NY Times which I lived in NYC and comes from Paula Wolfert who also uses a freeze-dried starter called Solait. At least she did when the recipe was printed - don't know if they make it any more.