Combine the buttermilk and cream in a saucepan and heat only to tepid (not more than 85 degrees on an instant reading thermometer). Pour into a clean glass jar. Partially cover and let stand at room temperature (between 65 and 75 degrees) for 8 to 24 hours, or until thickened. Stir and refrigerate at least 24 hours before using. The cream will keep about 2 weeks in the refrigerator.