Prepare the meat by breaking it up with a fork or chopping it lightly, then mix in the scallions, soy sauce, oil, salt, and Sichuan Peppercorn Powder until evenly distributed. Set the filling aside.
Divide the prepared dough into quarters and lightly flour a work surface. Working with one quarter of the dough at a time, knead the dough a couple of times and then use your hands to roll it into a rope about 1 1/2 inches thick. Cut the rope into six, small, even pieces.
Take one small piece of dough, roll it into a ball in your palms, and then use the heel of your hand or the bottom of a measuring cup to smash it into a flat, even circle. Use a dumpling rolling pin or wooden dowel to further flatten the circle into a 3 1/2 inch wrapper with a thick center. The best way to do this is to use the rolling pin on just the bottom third of the circle, then turn the dough counterclockwise a bit and repeat; this way the edges of the wrapper will receive even pressure, while the center stays untouched. (The finished wrapper should have the shape of a very flat flying saucer or fried egg.) Repeat with the remaining dough, setting the wrappers aside on a floured surface and making sure they don't touch each other.
When all the wrappers have been prepared, fill momos: place a wrapper in the palm of one hand and top with a small mound of filling (approximately 1 tablespoon). With your other hand, bring the right edge of the wrapper up toward the center of the filling. With your thumb on the edge of the wrapper, use your index finger to grab the edge of the wrapper about 1/2 inch from your thumb, then pinch the two parts of the edge together, to create a small fold. Repeat the pinching motion, bringing more and more of the dumpling's edge into the middle, and rotating the dumpling in your hand, creating pleats all around the dumpling. (The dough is flexible, so if you need to, pull and stretch the dough over the filling.) When all the edges of the wrapper have been secured together, pinch the top of the dumpling, where they meet, to ensure that they are stuck together well.
Fill a large pot that will fit under the steamer with water and bring it to a rolling boil. Place the momos on the steamer lined with cabbage leaves and steam them over the pot for 8 to 10 minutes, until the dough looks slightly translucent. (If using the steamer with stacked trays, the momos on the bottom may be done before those on the upper levels.)