The Accidental Locavore sometimes has leftover buttermilk from making biscuits and unlike my mother, drinking it, is not for me. However, crème fraîche is the answer and it couldn't be easier.
Mix the buttermilk and cream in a glass container. Cover and let it stand at room temperature for 8-24 hours until very thick. Refrigerate. It will keep (and get a bit thicker) in the refrigerator for up to 10 days. Serve and enjoy.