Creme fraiche is similar to sour cream but milder and sweeter, with a touch of nuttiness. Unlike sour cream, it can be simmered in sauces without curdling. Look for creme fraiche at well-stocked supermarkets or at gourmet food stores. To make your own, combine 1 cup of nonultrapasteurized heavy cream and 1 Tbs. buttermilk in a saucepan and heat over medium-low heat until just lukewarm. Pour into a bowl, cover loosely and let stand at room temperature until thickened, 8 to 48 hours or more. Chill before using.