Pour the whipping cream into a small saucepan, and set over low heat. Stir until the chill is off; do not heat above 100 degrees (lukewarm). Remove pan from the heat, and stir in the buttermilk. Pour the liquid into a clean glass jar.
Place the lid on the jar, without tightening it, and place the jar in a warm (80 degrees to 90 degrees) spot. Let the cream develop until it is noticeably thicker, 12 to 24 hours. Stir gently, and refrigerate at least 4 hours to overnight, chilling and thickening thoroughly. Cover, and refrigerate up to 10 days.