Well, my curiosity got the better of me and creme fraiche went onto my grocery list. You can imagine my shock when I saw the price of it (at least at our market) was $7+ per cup!!! That's just crazy.
So in my
2 cups of heavy whipping cream (at least 45% butterfat)
3 tablespoons buttermilk (OR) plain yogurt (see note)
Mix the two ingredients together well, and put it into a very clean glass jar. Cover it loosely and let it sit on your kitchen counter for 24 hours (room temperature should be 70° to 75°).
It's good if you give it a quick stir after 8 hours, but that isn't absolutely necessary.
After 24 hours, it will still seem a little thin, but that's OK. Put a lid on the jar and put it in the fridge for another 24 hours. After 24 hours in the fridge, it will thicken up and be ready to use in any recipe that you would normally use sour cream for.
People say it is the "mild cousin" to America's commercial sour cream, but I think it is much tastier than that. Of course, the flavor of your creme fraiche will depend on the "flavor" (quality) of whipping cream, and brand (quality) of buttermilk.
Creme fraiche is excellent for baking sweet AND savory dishes. It is much more tolerant to heat (than commercial sour cream), so it works well in gravy or sauces and makes killer mashed potatoes!! Sweetened, it is nice over any dessert, especially fresh fruit. Can you tell that I am a recent convert??
NOTE: Many of the recipes for creme fraiche (that are online) insist on using UN-pasteurized cream, but I couldn't find it, so I just used regular pasteurized heavy whipping cream and it worked fine.
NOTE: The buttermilk or yogurt that you use HAS to have live cultures in it. The buttermilk at our market was low fat and didn't mention "live cultures", so I opted for the same amount of organic unflavored yogurt with live cultures and it worked very well.