For homemade creme fraiche, heat cream and buttermilk in a saucepan to 65 degrees. Check with thermometer.
Pour mixture into clean screw-top jars and cover loosely with plastic wrap. Leave in a warm place for 24 hours, stir well once and then store in the refrigerator.
After 2-3 days, proper creme fraiche will have formed. The thickness will increase day by day. In the refrigerator homemade creme fraiche can be kept up to 14 days.