My culinary school instructor, Chef P, used the following ratio to create his crème fraiche: 2 tablespoons to 1/3 cup buttermilk for every 1 cup of whipping cream. I can still hear him muttering “Sour cream? What is this sour cream?” in his heavy french accent as he mixed together the cream and buttermilk.
Combine in a glass jar or bowl, cover, and let stand at room temperature for 24-36 hours, until thickened. Keep in the fridge for up to 10 days.
I’ve found that 3-4 tablespoons of buttermilk in 1 cup of whipping cream yields a thick, luscious, tangy cream perfect on top of berries, whipped into buttercream icing, or spiked with dill & lemon and served with potato pancakes.