A Bulgarian Yoghurt recipe by Rumi Radenska
Recipe from the Traditional Greek Cooking by George Moudiotis. Greek style yogurt is thicker and creamier with a higher fat content than typical yogurts found in the U.S. Typical fat content of the cow's milk yogurt is 9% as opposed the U.S. yogurts are less than 4% fat.
Quark is similar to sour cream. You can make your own using purchased buttermilk.