Prepare masa according to package directions. In a mixer, combine prepared masa, shortening, baking powder, and a pinch of salt.
Boil the chiles in one pot with water and soak for a few hours until they are very soft. In another pot of water, simmer the pork shoulder with garlic, black pepper, and cumin. Once the pork is tender, shred the meat with two forks.
Puree the cooked chiles in the blender and add some of the stock from the pork shoulder.
Add 1 Tbs. lard to a round pot. When lard is very hot, add the chile puree, and cook until it's two to three shades darker. Add the pork stock and the shredded pork to the chile mixture.
Spread 2 Tbs. of masa on the smoother side of the husk, on the bottom two thirds of the leaf. (The tapered end shouldn't contain masa.)
Place 1 tsp. pork filling in the middle of the masa, and fold the two sides over. Then fold over the tapered end.
Place the tamales in a pot with a small amount of water and steam for about 1 1/2 hours.