In a stainless-steel or glass bowl, whisk together 2 cups cream and 2 Tablespoons buttermilk or sour cream until blended.
Transfer to a plastic or glass container with an airtight lid and cover the container.
Let the cream stand at room temperature (about 68F degrees) until it thickens.
This will take 24 to 48 hours. Once it is ready, store in the refrigerator for several weeks.
As the creme fraiche ages in the refrigerator, it will give off some watery liquid. Spoon it off and discard it to keep the creme fraiche thick.