Set the heavy cream out to bring it to room temperature.
Combine heavy cream and buttermilk in non-reactive container. Cover with cheese cloth to let it "breath". Allow it to rest at room temperature until thickened to desired texture, about 12 hours.
If you like a thicker Creme Fraiche, you can let it sit out for up to 24 hours.
Store in the refrigerator. It will keep for about 2 weeks.