http://allrecipes.com/recipe/222387/chef-johns-creme-fraiche/
Combine cream and buttermilk in a glass jar. Cover tightly with cheesecloth (or any breathable material) and let sit at room temperature (70 to 75 degrees F (21 to 24 degrees C)) for 24 hours. Stir, screw on lid, and refrigerate for 24 hours before using.