From around my french table.
This takes 2 days, so if you need it sooner, make other plans!
1 C heavey cream
1 Tb buttermilk or plain yogurt
Put cream and buttermilk in a clean jar with a tight-fitting lid. Shake vigorously for a minute or so. Put jar on counter and leave for 12-24 hours or until cream thickens slightly. (The warmer the room, the quicker the thickening.)
Once thick, transfer to the fridge and let chill for a day before you use it.
(Keeps for about 2 weeks, getting tangier as time goes by.)