Mix 1/4 cup buttermilk with 2 cups whipping cream. Cover with plastic wrap and put in warm place for 24 hours. It will not go bad. Check if thickened. If it is, put in fridge for up to 2 weeks. If not, put in warmer place until it does get thick. From Regan Daley's "In The Sweet Kitchen".
USED TO HAVE A RECIPE TO MAKE CREME FRAICHE, IT WAS WONDERFUL