An ever-growing number of restaurants are putting it on their menus and it is more frequently called for in recipes than it was even a year or two ago, but crème fraîche is still not an easy-to-find ingredient, nor is it one that everyone is familiar with. Crème fraîche is a thickened cream, with a slight tang and a texture that is somewhere between sour cream and whipped butter. In France, the cream is made with unpasteurized cream and is allowed to thicken naturally with bacteria already present in the milk. In the US, the cream must be pasteurized (heated to kill bacteria), so crème fraîche is made by adding a small amount of buttermilk or sour cream to get the thickening process started, then it is pasteurized again to kill the bacteria before sale.
Sour cream makes a good substitute for crème fraîche in most recipes, but unlike crème fraîche, sour cream can break or curdle when exposed to high heat. Fortunately, it is easy to make a version of crème fraîche at home. Simply add 2 tbsp buttermilk to 1 cup of heavy cream and let the mixture stand at room temperature for 8-24 hours, until thickened, before refrigerating.