In a small bowl with warm water, soak shiitake mushrooms until softened. Using hands, squeeze liquid and then mince.
In a bowl, combine ground pork, cornstarch, and water. Mix thoroughly until very sticky.
Add shiitake mushrooms, water chestnuts, spring onions, shrimp, ginger, sesame oil, cooking wine, soy sauce, salt, pepper, sugar, and Maggi chicken cube. Using hands, mix thoroughly until well blended and very sticky.
Place a wrapper over your hand, spread about a tablespoon of the mixture onto the wrapper, then place it between your thumb and index finger to form a cup. Using a bread knife or a small spatula, push the filling down as you turn the siomai to wrap around the filling.
Garnish with green peas and/or chopped carrots. Repeat with the remaining mixture and the wrapper.
Arrange in a single layer on a parchment-lined baking sheet. Cover with film and freeze for 20-25 minutes.
Brush steamer basket with oil. Arrange siomai in a single layer and steam for 10-12 minutes.
Gently remove from steamer and serve hot with dipping sauce.