Preparing the filling. In a large blender or food processor (or working in batches), combine the chiles, garlic, pepper and cumin. Add 3 cups water, cover and blend to a smooth puree. Strain the mixture through a medium-mesh strainer into a medium-size (3-quart) saucepan.
Add the meat, 3 cups water and 1 teaspoon salt. Simmer, uncovered, over medium heat, stirring regularly, until the pork is fork-tender and the liquid is reduced to the consistency of a thick sauce, about 1 hour. Use a fork to break the pork into small pieces. Taste and season with additional salt if necessary. Let cool to room temperature.
Preparing the batter. With an electric mixer on medium-high speed, beat the lard or shortening with 2 teaspoons salt and the baking powder until light in texture, about 1 minute. Continue beating as you add the masa (fresh or reconstituted) in three additions. Reduce the speeded to medium-low and add 1 cup of the broth. Continue beating for another minute or so, until a ½-teaspoon dollop of the batter floats in a cup of cold water (if it floats you can be sure the tamales will be tender and light).
Beat in enough additional broth to give the mixture the consistency of soft (not runny) cake batter; it should hold its shape in a spoon. Taste the batter and season with additional salt if you think necessary.
For the lightest textured tamales, refrigerate the batter for an hour or so, then rebeat, adding enough additional broth or water to bring the mixture to the soft consistency it had before.
Preparing the banana leaves. Unfold the banana leaves and cut off the long, hard sides of the leaves (where they were attached to the central vein). Look for holes or rips, then cut leaves into unbroken 12-inch segments (you will need 20). Either steam the segments for 20 minutes to make them soft and pliable, or one at a time pass them briefly over an open flame or hot electric burner until soft and glossy.
Setting up the steamer. Steaming 20 leaf-wrapped tamales can be done in batches in a collapsible vegetable steamer set into a large, deep saucepan (if you stack the tamales more than two high they will steam unevenly). To steam the whole recipe at once, you’ll need something like the kettle-size tamal steamers used in Mexico or Asian stack steamers, or you can improvise by setting a wire rack on 4 coffee or custard cups in a large kettle.
It is best to line the rack or upper part of the steamer with leftover scraps of banana leaves to protect the tamales from direct contact with the steam and to add more flavor. Make sure to leave tiny spaces between leaves so condensing steam can drain off.
Forming the tamales. Cut twenty 12-inch pieces or string or thin strips of banana leaf. One at a time, form the tamales: Lay out a square of banana leaf, shiny-side up, and spread 1/3 cup of the batter into an 8x4-inch rectangle over it (as shown in the illustration). Spoon 2 tablespoons of the filling over the left side of the rectangle of batter, then fold in the right third of the leaf so that the batter encloses the filling. Fold in the uncovered third of the leaf, then fold in the top and bottom. Loosely tie the tamales with string and set them in the steamer.
Steaming and serving the tamales. When all the tamales are in the steamer, cover them with a layer of banana leaf scraps or leftovers. Set the lid in place and steam over a constant medium heat for about 1 ¼ hours. Watch carefully that all the water doesn’t boil away and, to keep the steam steady, pour boiling water into the pot when more is necessary.
Tamales are done when the leaf peels away from the masa easily. Let tamales stand in the steamer off the heat for a few minutes to firm up. For the best textured tamales, let them cool completely, then re-steam about 15 minutes to heat through.
Working Ahead: Both filling and batter can be made several days ahead, as can the finished tamales; refrigerate, well covered. Re-steam (or even microwave) tamales before serving. For even more flexibility, batter, filling or finished tamales can be frozen. Defrost finished tamales in the refrigerator overnight before re-steaming.