These little pork-and-shrimp meatballs are rolled in raw rice, then steamed until the grains resemble shiny pearls.
Soak shrimp, if using, in just enough hot water to cover, until soft, about 30 minutes; drain. Place sweet rice in a bowl and cover with 3 cups water. Let stand about 30 minutes. Drain and set aside on a plate.
Place pork in a large bowl and stir in 2 tablespoons of cold water. Add shrimp, scallion, ginger, soy sauce, salt, cornstarch, and egg and mix until well combined.
Dampen hands with cold water and roll 1 tablespoon of the pork mixture in palms to form a 1 1/4-inch-diameter ball. Roll the ball in the rice to completely cover with grains. Continue, using all the pork mixture.
Fill a wok with 1 inch of water. Place a small heatproof bowl upside down in the center and then place a plate on top of the bowl, making a steaming rack. Cover the wok and bring the water to a boil. Place 12 to 14 pearl balls on the plate and steam, covered, until rice is cooked, about 20 minutes. Alternatively, fill a bamboo steamer with pearl balls and steam over the wok. Serve immediately.