Heat the cream and buttermilk in a saucepan over low heat until it reads about 85°F on your thermometer. (This can also be done in the microwave, but be careful to heat slowly, you don't want it to go above 85°F. )
Pour the mixture into a nonreactive bowl and cover with plastic wrap. Place the bowl in a warm spot in your kitchen (warm, but not raging, radiator is perfect) for about 8 hours, or until it is nice and thick.
Place in the refrigerator and allow to cool thoroughly for several hours. The crème fraîche will be nice and thick and ready to use. If it is thicker than you would like, you can simply stir it and the texture will loosen up, or you can "lighten" it with a bit more cream. I don't usually sweeten the crème fraîche, but you can certainly stir in a bit of sugar or honey to taste.