You'll want to seek out a good quality heavy cream that is pasteurized, not ultra-pasteurized. If you can only find ultra-pasteurzed, it will work, but will take longer to thicken.
To start, you want to pour 1 cup of heavy cream into a non-reactionary container (basically, any container that isn't made from iron or aluminum).
Next, add two tablespoons of buttermilk to the heavy cream. Cover the bottle with a lid and shake until everything is thoroughly combined.
Loosely cover the heavy cream mixture and allow it to sit out on your kitchen counter for 12-24 hours. Ideally the temperature in your kitchen will be from 72 to 78 degrees. My apartment tends to be on the cooler side, so it always takes mine a full 24 hours to thicken. After it's at the perfect consistency, transfer it to your fridge. The creme fraiche will be good for up to 2 weeks.
I'm the writer behind the food blog, A Cozy Kitchen. My blog is a place where my love for cute things, pancakes and corgis is celebrated. In Spring 2013 my first cookbook, PANCAKES: 72 Sweet and Savory Recipes for the Perfect Stack, was published.