Quickly rinse mushrooms in water, pat dry and twist stems loose or separate them from the caps with the tip of a knife, set aside. In a medium bowl, combine shrimp, minced scallions, ginger powder, waterchestnuts, sherry and 1 tablespoon each of sugar, soy sauce and sesame oil; to form a paste. Stuff each mushroom with a teaspoon or more of the shrimp paste (depending on the size of the mushroom), bake in a 400º oven for 30 minutes, draining excess liquid every 10 minutes. Combine remaining sugar, soy sauce sesame oil, the chicken stock and oyster sauce in a small saucepan and bring to a boil, pour sauce over stuffed mushrooms.