Place the husks in a large bowl and pour boiling water over them. Let the husks soak until soft and pliable, about 2 hours. Drain the husks and select 24 pieces, each 5 to 6 inches wide, as wrappers; keep the remainder handy. Cover both the wrappers and the remaining husks with dampened paper towels to keep moist.
Lightly toast the chiles on both sides by pressing them against a heated skillet with a spatula for a few seconds. Transfer the chiles to a small bowl, cover with 1 cup of boiling water and soak until soft, about half an hour. In a blender or food processor, puree the chiles with 1/2 cup of their soaking liquid.
In a medium pot, cover the pork, onion and garlic with water. Add the coarse salt and pepper, and simmer, partly covered, until the pork is tender, 1 1/2 to 2 hours. Drain the pork, reserving the broth, and shred the meat with a fork. Add half of the pureed chiles to the meat and stir in enough broth to form a moist, spreadable mixture. Taste the filling and adjust the seasoning.
Combine the masa harina with the baking powder and salt in a large bowl. Stir in the 31/4 cups of hot water, the lard and the remaining pureed chiles and mix together with a wooden spoon or your hands until a soft, mushy dough forms. (It should be moist but not runny, and it should not stick to your hands.)
Using a tablespoon dipped in water, spread the masa over the corn-husk wrappers, leaving a 2- to 3-inch border on the tapered (bottom) end of the husks and about 1 inch on the remaining sides. Top the masa with about a tablespoon of filling, leaving a 1/2 -inch border of masa all around. Pull the sides of the husk together to seal, then fold up the tapered end and tie the package shut with a thin strip of husk (the top remains open). If the husk is too narrow to close, add another piece to cover the seam.
Pour an inch of water into a saucepan, place the collapsible steamer inside and invert the heatproof mug in the center of the steamer. Arrange the tamales around the mug, open end up. Layer the tops of the tamales with the remaining husks, then cover with a lid (trim the tops of the wrappers, if necessary). Bring the water to a simmer and steam the tamales for about 1 1/2 hours, adding more water as necessary, until the husks peel away from the dough.