In a small saucepan, heat the cream until it is just warm to the touch (about 100° F or 40°C). Remove from the heat and stir in the buttermilk. Pour the mixture into a clean jar and place the lid on top without screwing it down.
Place the jar in a warm corner of the kitchen and let stand for 24 hours, stirring a couple of times during this time. At the end of 24 hours the cream should be somewhat thickened. Place in the refrigerator with the lid screwed down and let sit another 24 hours, after which time it should be fairly thick.
You can whip the cream to thicken it even further. That's it. Your creme fraiche will last about 10 days in the refrigerator.