1. In a food processor, combine the shiitakes, seitan, button mushrooms, cornstarch, mirin, sugar, tamari, sesame oil, and salt. Buzz to a paste, pausing from time to time to scrape down the sides. Scrape the mixture into a bowl and stir in the scallions, snow peas, and jícama. Let stand to blend flavors, about 30 minutes.
2. Set the wrappers on your work counter, covering with a slightly damp clean kitchen towel. Have the filling handy, and a few plates that have been sprayed with cooking spray on which to place the finished dumplings.
3. Place 2 teaspoons to 1 tablespoon of the filling mixture smack dab in the middle of a round wrapper. Draw the edges up around but not over the filling on all sides, pinching in a little, moistening the upper edge, by moistening your thumb and forefinger into water as you pinch and pleating the edges (see what I meant about the drawstring bag effect?). Flatten the bottom a bit, so that the dumplings can stand upright. Please note that you should see a good little bit of the filling—an inch or so. Repeat until filling or wrappers or both (if you’re lucky!) are used up.
4. Press 3 peas into the top of the exposed filling. Steam the dumplings in a metal steamer or right on the plate. This works if you don’t have a metal steamer, but do have a pot, wok, or Dutch oven large enough to accommodate the dumpling plate with at least 1 inch of space between the edge of the plate and the pot, through which the steam can rise. Place an upside-down heatproof bowl or empty can in the bottom of the pot to act as a pedestal for the dumpling plate. Pour in water to a depth of about 2 inches or whatever’s adequate so that the bowl or can will project slightly above the water level. Set the dumpling-filled plate on top of the bowl or can. Cover the pot and bring the water to a boil over high heat. Steam, tightly covered, until the filling’s hot and the wrapper is soft, 15 to 20 minutes.
5. Remove the shui-mai from the steamer and poke a cilantro leaf onto the top of each one. Serve hot, with dipping sauce.
For the Dipping Sauce: Combine all of the ingredients. Place on the table in small bowls so that each guest has his own private dipping stash. If making a large batch, cover and refrigerate.