Whereas boiled dumplings have crinkly skins that are tender from having been immersed in hot water, and panfried dumplings have skins that are a combination of crisp bottom and chewy top, steamed dumplings are texturally in between—the skins are slightly chewy and soft. Gentle steam heat also protects the filling flavor and you taste it more, a bonus for delicate combinations such as this one.
Unlike many other Chinese dumpling fillings, this vegetarian filling is cooked first to allow the cornstarch to work its cohesive magic, binding the vegetables and pressed tofu together so that they are easier to control when you are assembling the dumplings. I like to use some of the liquid from soaking the mushrooms to season the filling with an earthy savor; if you do too, remember to strain the heady liquid through cheesecloth or paper towel to remove any bits of grit.
1To make the filling, put the spinach in a large bowl. Bring a kettle of water to a boil and pour a generous amount over the spinach. Let the spinach wilt for about 30 seconds, drain, rinse with cold water, and drain again. To remove excess moisture, squeeze the spinach in your hands over the sink. When you are done, there should be about 1/2 cup firmly packed spinach.
2In a bowl, combine 1/4 cup of the reserved mushroom soaking liquid, salt, white pepper, sugar, soy sauce, and sesame oil. Stir to dissolve the sugar. Set this flavoring sauce aside.
3In a wok or large skillet, heat the canola oil over medium heat. Add the ginger and stir-fry for about 30 seconds, until aromatic. Add the spinach, carrot, mushrooms, and pressed tofu. Stir to combine and then pour in the flavoring sauce. At first all the liquid will seem to have been absorbed, but after 2 minutes, there will be a little bubbling liquid in the skillet. At that point, give the cornstarch mixture a final stir and stir it into the filling. When the mixture thickens, turn off the heat and add the Chinese chives. Transfer to a bowl and set aside to cool completely before assembling the dumplings. You should have about 2 cups of filling. (The filling can be prepared 1 day in advance and refrigerated. Bring it to room temperature before assembling the dumplings.)
4Form 16 wrappers from half of the dough. Aim for wrappers that are about 3 1/4 inches in diameter.
5Before assembling the dumplings, line steamer trays or a baking sheet with parchment paper and set aside. (If you are making the dumplings in advance, or freezing them, lightly dust the parchment paper–lined tray with flour to avoid sticking.)
6To assemble the dumplings, hold a wrapper in a slightly cupped hand. Scoop up about 1 tablespoon of filling with a bamboo dumpling spatula, dinner knife, or fork and position it slightly off-center toward the upper half of the wrapper, pressing and shaping it into a flat mound and keeping about 1/2 to 3/4 inch of wrapper clear on all sides. Then fold, pleat, and press to enclose the filling to create a half-moon, pea pod, big hug, or pleated crescent shape. If you are steaming right away, place the finished dumpling in a steamer tray, sealed side up and 1 inch away from the edge if you are using a metal steamer. Repeat with the other wrappers before forming and filling wrappers from the remaining dough, keeping the finished dumplings covered with a dry kitchen towel as you make the rest. If you don’t have enough space on your steamer trays to steam all the dumplings at once, or if you are not steaming them right away, place the waiting ones on the prepared baking sheet spaced a good 1/2 inch apart.
7Once all the dumplings are assembled, they can be covered with plastic wrap and refrigerated for several hours; they can be cooked straight from the refrigerator. For longer storage, freeze them on the baking sheet until hard (about 1 hour), transfer them to a zip-top freezer bag, pressing out excess air before sealing, and keep them frozen for up to 1 month; thaw completely on lined steamer trays, using your finger to smooth over any cracks that may have formed during freezing, before steaming.
8To cook, steam the dumplings over boiling water for about 8 minutes, or until slightly puffed and somewhat translucent. Remove the trays and place each atop a serving plate.
9Serve immediately with the dipping sauce, either in a communal bowl with a spoon or portioned into individual bowls or dipping sauce dishes. As with all jiǎozi, it is easiest to eat these with chopsticks in one hand and soupspoon or rice bowl in the other, angling the bowl or spoon to catch any drips.