Heat 2 cups of heavy cream to 100° F. Mix in 2 tablespoons buttermilk or sour cream. Place in a jar, cover, and let sit at room temperature for six to eight hours. Chill until thickened. The mixture will keep, refigerated, for 2 to 3 weeks. It makes a delicious topping for fruit pies and other desserts, a garnish for soups, and can even stand in for clotted cream at afternoon tea. Can also combine with vanilla extract or honey.