To make creme fraiche, mix 1 cup of heavy cream with 2 tablespoons of buttermilk and cover the mixture with a clean cloth. Allow the mixture to sit in a warm room for 12 to 16 hours until it has thickened but can still be poured. Stir and refrigerate it.
The acid within the mixture prevents bacterial growth as it thickens on the counter, and it can be safely refrigerated for up to one week. If you are using whipping cream, use a glass bowl and let the mixture sit at room temperature for eight to 24 hours. Creme fraiche made with whipping cream can be stored in the refrigerator for up to 10 days.
Creme fraiche is versatile, as it can be used as an ingredient or topping for a variety of different dishes such as a base ingredient for various dips and sauces, toppings for soups, and even in desserts where the cream is whipped and sweetened. The cream does not curdle when cooked at high temperatures, and it does not separate when combined with cooking wine or vinegar, which is common with other creams.
The most common way to use creme fraiche is as a rich substitute for sour cream. Vermont Creamery suggests using the cream to make gratin potatoes and even brownies.