Grate potatoes and set in a mesh strainer over a large bowl. Allow to drain, pressing occasionally, 30 to 60 minutes. The starch will sink to the bottom of the liquid in the bowl. Drain off the liquid, and reserve the starch to mix back in with the batter.

Combine eggs, yolk, potatoes, starch, onions, flour, baking powder, parsley, salt, and pepper in a large bowl.

Heat ¼ cup duck schmaltz in a large frying pan. Fry until golden, flip carefully, then transfer to paper towel-lined plates to drain. Can be kept warm for up to 30 minutes in a 200°F oven. Add more fat, adjusting heat as pan warms up, to fry remaining latkes.