How to Make It
Peel the shrimp; place the shells in a medium saucepan. Toss the shrimp in a bowl with the salt and white pepper, cardamom and nutmeg; set aside.
Add the stock to the shells and boil for 5 minutes. Strain, reserving the stock; discard the shells. Add the water chestnuts to the stock and simmer for minutes. Remove the water chestnuts with a slotted spoon and set aside.
Add the cream to the stock and boil until reduced to 1/2 cup, about 1 minute. Mince enough of the white part of the scallions to measure 2 tablespoons. Mince enough of the scallion greens to measure 1 tablespoon.
In a medium skillet, heat the butter over moderate heat. Add 2 tablespoons of minced scallion whites and cook, stirring, until softened, about 1 minute. Add the shrimp and chestnuts and toss to combine. Add the cream-stock mixture, reduce the heat and simmer, stirring, until the shrimp turn opaque, about 5 minutes.
In a small bowl, stir the sherry into the cornstarch. Add to the shrimp and stir over moderate heat until the sauce boils and thickens. Season with salt and pepper to taste. Spoon into a warmed serving dish and sprinkle with 1 tablespoon of scallion greens and serve hot.