Combine the cream and buttermilk or sour cream in a glass jar. Add the lid, shake for 10-15 seconds, then remove the lid and allow the mixture to sit at room temperature (around 21C/70F) for 8-24 hours. The longer you allow the mixture to sit, the thicker it will become.
The creme is finished when it has reached its desired thickness. Add the salt, stir to incorporate and refrigerate. You mixture may need to refrigerate the creme before it sets completely. Lasts for a 7-10 days in the fridge.
Recipe by Thanks For The Food at https://thanksforthefood.com/homemade-creme-fraiche/