Crème fraîche is fermented cream thickened by acid-producing bacteria. Its nutty, slightly sour flavor and smooth consistency make it especially good for enriching sauces and for using as a garnish for desserts, canapés, soups, and caviar.
2 cups heavy cream
1/4 cup buttermilk
It's simple to make crème fraîche at home.
In a small saucepan, warm two cups of heavy cream (avoid ultra-pasteurized cream if you can; because of the processing, it doesn't ferment as well) to 100°F. If you don't have a thermometer, you can test the temperature with your finger: the cream should feel slightly warmer than body temperature. Stir in 1/4 cup store-bought buttermilk (use the freshest buttermilk possible to ensure souring and thickening) and then transfer the mixture to a clean glass or plastic container with a cover.
Set the cream in a warm spot that's about 70° to 80°F (such as the top of a gas stove, providing the pilot is lit, or near a heater) and let it stand for at least 24 hours.
Put the crème fraîche in the refrigerator for at least four hours to let it thicken further.
Crème fraîche will keep well under refrigeration for about ten days.
Source of recipe: Elaine Sterling, Fine Cooking #27