A dairy product made from whipping cream and a bacterial culture, which causes the whipping cream to thicken and develop a sharp, tangy flavor. If you can't find crème fraîche in your supermarket, you can make a substitute by combining 1/2 cup whipping cream (do not use ultra-pasteurized cream) and 1/2 cup dairy sour cream. Cover the mixture and let it stand at room temperature for two to five hours or until it thickens. Cover and refrigerate for up to one week.