Mix dumpling flour, baking powder and dumpling salt in a bowl. Rub in the cold butter until fine crumbs form, then stir in buttermilk just until a firm dough comes together; cover and rest for 10 minutes.
Melt soup butter in a wide 6-quart pot and cook leek and parsnip for 6 minutes. Add broth, soup salt and pepper and simmer for 12 minutes until parsnip is nearly tender.
Dust a board with the rolling flour and roll dough about 1/8 inch thick. Cut into strips about 1 by 2 inches, shaking off excess loose flour.
Lower strips into the gently simmering broth one at a time, separating them with a spoon. Simmer uncovered for 12 to 15 minutes, stirring gently, until the thickest dumpling has no floury center.
Add turkey and simmer for 5 to 8 minutes until it reaches 165°F and dumplings are fully cooked. Stir in cream, warm gently for 1 minute without boiling hard and serve.