Add heavy cream and buttermilk to a glass jar that has a tight fitting lid, and whisk to combine. Cover the jar with a paper towel, and secure with a rubber band, string or canning ring.
Let sit at room temperature for 12 - 24 hours. Completion time will vary based on the temperature of your kitchen, and personal taste. Check for consistency at 12 hours. Simply dip a spoon in the mixture, then pull it out to see how thick it is. The Crème Fraîche should be thick and creamy, but not completely solid.
Once you've achieved the desired consistency, remove the paper towel and clamp on the lid. Store the Crème Fraîche in the refrigerator for 2 to 3 weeks. Sometimes longer!