1. 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.

2. 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.

3. 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.

4. 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.