Trim green onions. Cut and separate white part from green. Finely chop the green part and set aside. Rough chop white ends.
Transfer the ground pork, half of the shrimp, white part of green onions, soy sauce, sherry, sesame oil, ginger, lemongrass, garlic, sugar, Chinese 5 Spice powder, salt and white pepper to a food processor and pulse several times until ground and well combined. Using a spatula scrape down sides of bowl and pulse again until a smooth paste forms.
Scoop the mixture into a medium bowl, and using a spatula, fold in remaining chopped shrimp, water chestnuts and chopped green onion tops.
Place a 2 tablespoon scoop of the mixture into the center of a wonton wrapper and wet the edges with your finger with water. With your hands gather the sides of the wrapper up and around the filling, letting the wrapper pleat, squeezing the wrapper gently to seal and tap the dumpling to flatten the bottom so that it can sit upright. Repeat until all the filling is used. This should make about 30 dumplings.
Place these on tray sprinkled with cornstarch to help them from sticking. Refrigerate uncovered for 30 minutes to firm up and make sure the wet wrapper pleats seal well and keep their shape (optional, see Note 2).
Fill your wok with 2 cups water. Set a bamboo steamer over wok lined with a circular cut piece of parchment paper or coat with cooking spray/oil. Bring water to a boil and place your Shumai in bamboo steamer about a half an inch apart. Cover with lid and steam until filling is cooked through, about 8-10 minutes. Serve immediately with dipping sauce.