How to Make It
Using tongs, briefly toast the guajillo chiles over an open flame or in a cast-iron skillet until fragrant, about 5 seconds per side. Transfer the chiles to a blender. Add 2 cups of the hot water and let stand for 15 minutes.
Add the garlic, chipotles, black pepper, cinnamon and oregano to the blender and puree until smooth. Strain the sauce into a large enameled cast-iron casserole, pressing hard on the solids. Add the pork to the casserole along with the remaining 6 cups of water and bring to a boil. Cover partially and simmer over low heat until the pork is tender, about 2 hours.
Using a slotted spoon, transfer the pork to a bowl and let cool slightly. Boil the sauce until it is reduced to 4 cups, about 20 minutes.
Shred the pork with 2 forks and return it to the sauce. Simmer uncovered until the sauce is reduced and just coats the pork, about 20 minutes. Season the pork with salt and let cool slightly.
In a large bowl, stir the masa harina with the water until evenly moistened; let the masa cool. In the bowl of a standing electric mixer fitted with the paddle, beat the lard with the baking powder and salt at medium speed until fluffy, about 3 minutes. With the machine on, add the masa in golf-ball-size lumps, then drizzle in the chicken stock and beat until completely smooth. Increase the speed to high and beat the masa until fluffy, about 3 minutes; the texture should resemble soft hummus.
Preheat the oven to 300° and grease a 10-inch cast iron skillet with lard. Spoon two-thirds of the masa into the skillet, mounding it slightly up the side. Spread 2 1/2 cups of the pork filling evenly over the masa and top with the remaining masa.
Bake for 30 minutes, until nearly set. Increase the temperature to 375° and bake until firm and lightly browned, about 35 minutes longer. Let stand for 30 minutes, then cut into wedges and serve.