Peel and quarter the potatoes. Peel and cut the parsnips into large chunks. In a covered large saucepan, cook potatoes and parsnips in a small amount of boiling, lightly salted water for 20 to 25 minutes or until tender; drain.
Mash potatoes and parsnips with a potato masher or beat with an electric mixer on low speed. Add butter, salt, and pepper. Gradually beat in enough of the buttermilk to make a light and fluffy consistency. Spoon into a serving bowl. Sprinkle with parsley. Makes 8 servings.