I love Creme Fraiche! I often use it as a substitute for sour cream in recipes. It has a more mild flavor than sour cream and goes great in so many dishes.
I cup Buttermilk
2 cups Heavy Cream
Mix these together and leave in a warm place (80-90F or 26-32C) for anywhere from 8-24 hours. The longer it sits, the more of a sour taste it will pick up. Experiment to get it to your taste.
The best thing about Creme Fraiche is that it can be whipped and used as a whipped topping! It also will not curdle if boiled.