In a jar with a lid, place whipping cream and buttermilk or sour cream; cover securely and shake 15 seconds. Set aside at room temperature for 24 hours or until very thick. Stir once or twice during that time. NOTE: Cream will thicken faster if the room is warm. If it turns out that it's too thin, try to keep it in a warm place and continue fermenting it.
Hint: Preheat oven at 200 degrees F. Then turn oven off and place the jar with the cream in the oven.
Stir thickened creme fraiche well. Refrigerate at least 6 hours before serving.
Cover tightly and store in refrigerator for up to 2 weeks.
* You want to start this process when your cream is still fresh. Do not use cream that has been in your refrigerator for a week. Use the freshest cream you can purchase. Pasteurized cream is fine, but not ultra-pasteurized.
Mock Creme Fraiche Recipe: https://whatscookingamerica.net/Sauces_Condiments/CremeFraiche.htm