Bring 1 inch of water to a boil in a large saucepan fitted with a steamer basket. Add potatoes, cover and steam until very tender when pierced with a fork, 20 to 25 minutes. (Check the water level near the end of steaming to be sure the pan doesn’t boil dry. Add more boiling water as needed.)
Meanwhile, combine yogurt, scallions, parsley, butter, salt and pepper in a large bowl
Mix the cooked potatoes into the yogurt mixture, breaking them up with a fork until they crumble apart and lightly clump together.