Chef John's Creme Fraiche
Making creme fraiche is very easy and, once you taste the magic of homemade sour cream, you'll have a hard time not repeating this esoteric exercise. Sure it takes a couple days, but the effort is minimal and the payoff is marvelous.
Be sure to use the best, freshest cream you can find. Look for pasteurized, not ultra-pasteurized heavy whipping cream. Make sure to use cultured buttermilk, otherwise you will wait a full day to see nothing happen.