Place the corn husks in a large heatproof bowl and cover with hot water. Let soak for at least 24 hours. Drain well before using.
In a medium pot, add the pork butt, onion, the halved head of garlic, and 1 tablespoon of salt. Cover with water and bring to a boil. Reduce the heat to low and simmer until the meat is tender, about 1.5 to 2 hours. Strain the pork, reserving the pork broth for later. Let the pork cool for at least 20 minutes, then finely chop. Finely chop the onion and garlic cloves and mix in with the cooked pork.
In a Dutch oven, combine the chopped pork with 2 cups of the pork broth, the chopped garlic, cumin, black pepper, and 1 tablespoon of adobo sauce. Season with salt. Simmer over moderate heat, stirring occasionally, until the sauce thickens slightly, about 15 minutes. Add more adobo sauce to taste. Let cool.
Knead the masa with the pork lard, the remaining 1 tablespoon of adobo sauce, and ½ tablespoon of the chile cascavel. Season with salt. Add 1 to 2 cups of pork broth until you have a smooth, cohesive masa paste.
Working one at a time, smear one heaping tablespoon of masa on the smooth side of a corn husk in the palm of your hand to create a 2 x 3-inch rectangle, leaving a 1-inch border on all sides. Place 1½ tablespoons of the cooked pork meat on top. Roll the husk crosswise, then fold up the base and fold down the pointed tip so the tamales are tight little bundles. Repeat with the remaining corn husks, masa, and pork. Stack the assembled tamales with their seams and points facing up, being careful not to crush them.
Fill a large pot with 2 cups of broth and set a steam basket inside the pot (the liquid should not touch the steamer basket. Stack the tamales on the steam basket, cover and simmer over low heat for 1 to 2 hours, or until the meat looks caramelized around the edges.