Add the heavy cream to a medium-size saucepot and heat to 100°.
Transfer the mixture to a glass jar or container.
Add buttermilk to the heavy cream and gently mix.
Wrap the top using cheese cloth and a rubber band and let sit at room temperature for 6-24 hours or until thickened.
Notes
Make-Ahead: You can make this up to 1 day ahead of time.How to Store: Cover and keep in the refrigerator for up to 2 weeksWhen cooking the cream, it can go as high as 110°.Make sure the room you are leaving your crème Fraiche in is not below 60°.