In a small saucepan heat whipping cream over low heat until warm (90 degree F to 100 degree F). Pour the cream into a small bowl. Stir in buttermilk. Cover and let the mixture stand at room temperature for 24 to 30 hours (do not stir) or until mixture is thickened. Store in a covered container in the refrigerator for up to a week. Stir before serving. Makes 1 cup.