http://table.mpr.org/recipes/ sauce_fraiche.html

• 1 to 2 tablespoons cultured buttermilk 2 cups heavy cream (pasteurized, not ultra pasteurized or sterilized, and with no additives)
  
  Combine the buttermilk and cream in a saucepan and heat only to tepid (not more than 85 degrees on an instant reading thermometer). Pour into a clean glass jar. Partially cover and let stand at room temperature (between 65 and 75 degrees) for 8 to 24 hours, or until thickened. Stir and refrigerate at least 24 hours before using. The cream will keep about 2 weeks in the refrigerator.
  
  

• Crème Fraiche
  
  Ingredients
  
  1 c whipping cream (or 18% cream)
  1 Tbsp buttermilk
  
  Preparation
  
  Stir cream and buttermilk together in a bowl. Cover with saran wrap and allow to rest overnight at cool room temperature (8-12 hours) til it is thick. Refrigerate to thicken further.
  
  A word of caution: yoghurt cannot be used in substitution for the buttermilk. The bacterial cultures are different and only the bacterial cultures in buttermilk will set the cream.
  
  We have also successfully made crème fraiche using 10% cream. It takes much longer (at least 24 hours) to set.