1. Preheat the oven to 210°C (410°F).
2. Peel the sweet potatoes and cut them into cubes.
3. In a saucepan, cover the sweet potatoes with plenty of cold water, add salt, and bring to a boil. Let simmer for 15 minutes.
4. Meanwhile, rinse the duck legs under hot water to remove excess fat (use a little bit of the fat to grease an ovenproof dish).
5. Remove the skin and bones from the duck legs. Shred the meat with your fingers into the ovenproof dish.
6. Sprinkle with shallots, parsley, and pepper.
7. When the sweet potatoes are tender when pierced with a knife, drain them and mash them with a fork, gradually incorporating the butter and then the milk.
8. Cover the duck with the mashed sweet potatoes, sprinkle with breadcrumbs, and bake for 10 to 15 minutes.