In a small bowl, combine the cream and buttermilk. Partially cover with a towel and let stand in a warmer spot of your kitchen until the cream tastes slightly sour and has thickened to a pudding-like consistency, anywhere from 1 to 3 days depending on the temperature of your kitchen.
Line a tamis with cheesecloth and rest it over a mixing bowl. Place the cream in the tamis to drain. The whey will drip into the bowl (you can reserve it for other uses, such as marinade), and the crème fraîche will remain in the tamis.
Store the crème fraîche in an airtight container in the fridge for up to 2 weeks.