1. Rinse duckling with running warm water; drain on wire rack in sink a few minutes. Slowly pour about 5 quarts boiling water in a thin stream over both sides of duckling so its skin becomes almost white. Drain duckling well; pat skin and body cavity dry with paper towels. Then rub body cavity with salt and sherry.
2. Rest a large wire rack on top edge of a medium-sized roasting pan; place duckling, breast side down, on rack. Refrigerate duckling, uncovered, until early that evening. Meanwhile, make Thin Pancakes to use when serving duck; refrigerate.
3. Early that evening, with pastry brush, brush duckling skin with maple syrup. Place duckling, breast side up, on same wire rack and refrigerate uncovered until ready to roast next day.
4. On serving day, about 5 hours before serving, preheat oven to 200°F. (yes, 200°F.). Remove roasting pan with duckling on rack from refrigerator; discard liquid in roasting pan. Place duckling on rack in roasting pan, in oven. Roast duckling 1 1/2 hours. Turn oven temperature to 325°F. and roast duckling, breast side down, 1 1/2 hours, then breast side up 1 to 1 1/2 hours longer, until skin turns golden brown and crisp. When duckling is done, place on large serving board or platter.
5. About 20 minutes before duckling is done, heat Thin Pancakes: In 5- to 6-quart Dutch oven in 3/4 inch water, place 3 inverted 6-ounce custard cups. Unwrap pancakes and place on large heat-safe plate; set plate on top of custard cups. Over high heat, heat water to boiling. Reduce heat to low; cover and steam pancakes 10 minutes or until soft and hot.