Mix dumpling flour, baking powder, and dumpling salt. Rub in cold butter to fine crumbs; stir in buttermilk just until firm dough forms. Cover; rest 10 minutes.
Melt soup butter in wide 6-quart pot; cook leek and parsnip 6 minutes. Add broth, soup salt, and pepper; simmer 12 minutes until parsnip nearly tender.
Dust board with rolling flour; roll dough 1/8 inch thick. Cut strips about 1 by 2 inches; shake off loose flour.
Lower strips individually into gently simmering broth, separating with spoon. Simmer uncovered 12 to 15 minutes, stirring gently, until thickest dumpling has no floury center.
Add turkey; simmer 5 to 8 minutes to 165°F and dumplings fully cooked. Add cream; gently warm 1 minute without hard boiling. Serve.