Preheat oven to 400°F Steam parsnips until tender, about 15 minutes.
Transfer to baking sheet; roast until light brown, about 45 minutes. Puree in processor.
Bring water, butter and salt to boil in heavy medium saucepan, stirring until butter melts. Reduce heat to low. Add flour all at once; stir vigorously until mixture forms ball, about 2 minutes.
Transfer hot paste to bowl of heavy-duty mixer fitted with paddle attachment. Add eggs 1 at a time, beating well after each addition.
Continue beating 1 minute. Add 2 cups parsnip puree to paste mixture (reserve any remaining puree for another use); beat to blend. Stir in pepper. (Dough can be made 2 hours ahead. Cover and let stand at room temperature.).
Preheat oven to 350°F Line large baking sheets with parchment. Transfer dough to pastry bag fitted with larger star tip. Pipe 2-inch-diameter rosettes onto sheets, spacing apart.
Bake gnocchi until crisp and beginning to brown, about 25 minutes. Using spatula, transfer gnocchi to plates. Top with parsley, if desired.