In a bowl, stir together cream and buttermilk. Cover with a lint-free kitchen towel and let stand at room temperature for about 24 hours or until thickened. It should smell fresh and nutty. (Discard if it develops any extremely sour or off aromas.)
Stir with a whisk to blend, transfer to an airtight container or cover bowl with plastic wrap and refrigerate until chilled, for about 8 hours, and use within 1 week.