Crème fraiche is a thickened cream product made by allowing bacteria to sour heavy cream. Its flavor is somewhat similar to that of sour cream, but it has a slightly different consistency (somewhat like a very soft cheese) and a less tangy flavor. If you do not need crème fraiche for several days, you can make your own by mixing 1 to 2 tbsp. of buttermilk with 1 cup of whipping cream. Heat this mixture until it is just barely warm to the touch. Store it in a covered (but not airtight) container at warm room temperature for 24 to 36 hours, then refrigerate for up to 10 days. If you need an immediate crème fraiche substitute, use sour cream.
Measure out an amount of sour cream equal to the amount of crème fraiche called for in your recipe.
Add the sour cream to the recipe at the same time and in the same manner as you would have added the crème fraiche.
Reduce the heat if your dish including the sour cream ever approaches a boil. Crème fraiche is significantly better at handling high temperatures than sour cream, which can curdle if it reaches a boil. Maintain the heat at a gentle simmer (at most) when substituting sour cream for crème fraiche.