A home version of crème fraîche can be made by adding some cultured buttermilk or sour cream, which contain cream-culture bacteria, to heavy cream (1 tablespoon per cup/15 ml per 250 ml), and letting it stand at a cool room temperature for 12 to 18 hours or until thick.
From the book On Food and Cooking (2nd edition) by Harold McGee. © 2004 Harold McGee.
By permission of Scribner, a division of Simon & Schuster, Inc.