Editor's Note: Hot bean paste is a savory, spicy combination of fermented soybeans and chiles. Look for jars at Asian markets.
For the filling, combine the pork, shrimp, soy sauce, vegetable oil, cilantro, scallion, mushroom, sesame oil, white pepper, and 1/3 cup water in a medium bowl. Mix well. Cover, and freeze for 1 hour (or refrigerate overnight) to firm the mixture and make it easier to handle.
Dissolve the cornstarch in 3 tablespoons cold water in a small bowl to make a paste. Line a baking sheet with waxed paper and sprinkle it with cornstarch. Moisten the edges of a dumpling wrapper by dipping your finger into the paste and running it over the edge of the wrapper. Using a blunt knife as a scoop, place about a tablespoon of the filling in the center of the wrapper. Bring the edges of the wrapper up to meet at the top of the filling and pinch them closed, squeezing the dough. Place the dumpling on the baking sheet. Repeat with the remaining filling and wrappers. (The uncooked dumplings can be made ahead, covered with plastic wrap, and refrigerated for up to 1 day. Or freeze them for up to 3 months. To freeze, place them in a single layer in a plastic storage container or plastic storage bag, arranged so that the dumplings don't touch one another. When you are ready to cook them, the frozen dumplings will be easy to remove, one by one.).
Bring a large saucepan of water to a boil over high heat. Add the dumplings and cover the pan. Cook until the filling is cooked through and the dumplings are floating on top of the water, about 4 minutes.
While the dumplings are cooking, make the sauce: Heat a wok or medium skillet over high heat. Add the oil and heat until it shimmers. Add the scallions, garlic, and black pepper, and stir-fry for 20 seconds. Transfer to a small bowl. Add the sugar, vinegar, hot bean paste, soy sauce, sesame oil, and hot chile oil to the scallions, and mix well. Divide the sauce among four soup bowls.
Carefully drain the dumplings in a colander. Place the dumplings on top of the sauce in the bowls, and serve immediately.