Crème fraîche is a relative of sour cream. It contains 30 percent fat compared with sour cream's 20 percent. While sour cream tastes sour, crème fraîche is rich and tart and tends to make other foods taste buttery. But unlike yogurt, crème fraîche isn't particularly acidic (so it's not great for marinades). and unlike sour cream, which curdles if heated too much, the higher fat content of crème fraîche means it won't separate. This makes it ideal for soups, sauces and simmered dishes.
It will, however, liquefy. That means that if you add it to the top of something, then toss it under the broiler, or even just dollop it onto something hot, it will melt. In France, where it originates, crème fraîche often is used in sauces for vegetables, particularly green beans and cauliflower, as well as in salad dressings, soups and pastries, and to top fresh fruit. It's sometimes used to make caramels and even is added to coffee and cocktails.
It's easy to make your own: Add a tablespoon of cultured buttermilk to 1 cup of cream and let it sit in a cool room for up to 24 hours, or until very thick. Refrigerate for several weeks. Crème fraîche is widely available at most grocers in the United States. It usually is found alongside the better cheeses, though it sometimes will be near the sour cream. It keeps, refrigerated, for about a month.