This thick dairy product resembles sour cream, but its flavor is much less tangy and more buttery. Unlike sour cream, it has the advantage of not curdling when heated. It is available at specialty stores and many supermarkets, but it can be pricey. To make your own crème fraîche:
Whisk together 1 cup heavy cream (not ultrapasteurized) and 2 tablespoons buttermilk in a saucepan. Heat over low heat just until the mixture is lukewarm. Transfer to a bowl and cover loosely with plastic wrap. Let stand in a warm place until it is about the consistency of heavy cream, 24 to 36 hours) the exact time depends on the room temperature). Transfer to a covered container and refrigerate for 24 hours to thicken more. It will keep in the refrigerator for up to two week.
from Tips Cooks Love, Rick Rodgers, Sur la Table