6 to 8 cups homemade chicken or pork stock, with the cooked meat shredded and
1 1/2 ounces Mexican chocolate, coarsely grated or finely chopped
1 thick slice day-old challah or brioche, crushed to fine crumbs
1 teaspoon salt
Zarela Martinez leaves in the veins of the chiles— the hottest part—but you can cut them away if you want to tone down the heat. Rinse the chiles under cold running water and shake off the excess moisture, but do not dry them. Heat a griddle or cast-iron skillet over moderately-high heat until a drop of water sizzles on contact. Place the chiles, a few at a time, on the griddle and let them heat, turning occasionally with tongs, just until the water evaporates and the chiles are fragrant. Allow between 30 to 45 seconds for the anchos, slightly less for the guajillos, which are very thin-skinned. The chiles should just become dry, hot and aromatic; do not allow them to start really roasting or they will have a terrible scorched flavor. Remove from the griddle as they are done. Place in a bowl and cover generously with boiling water. Let soak for at least 20 minutes, then drain.
In a small heavy skillet, cook the sesame seeds over moderate heat, stirring constantly, just until you see them starting to turn golden. Scrape the seeds out into a small bowl and set aside.
Grind the canela, cloves and peppercorns together in an electric coffee grinder or spice mill or in a mortar. In a medium skillet, heat 2 tablespoons of the lard over moderate heat until rippling. Add the ground spices and cook, stirring, just until fragrant, 1 to 2 minutes. Add the onion, garlic, tomatoes, plantain, thyme, oregano, raisins, almonds and sesame seeds. Cook, stirring frequently, for 15 minutes. Let cool for 10 minutes.
In a large Dutch oven or deep skillet, heat the remaining lard over moderately-high heat until rippling. Add the sauce, stirring well to prevent splattering. Stir in the remaining stock, a little at a time. Cover and cook, for 15 to 20 minutes, stirring frequently, until the chiles lose their raw edge. Stir in the bread crumbs and cook, stirring frequently, until the sauce is lightly thickened, about 10 minutes. Add the chocolate and cook, stirring constantly, until it is well dissolved. Add the salt and the shredded meat. Cover partially and cook, stirring occasionally, just until heated through, 7 to 10 minutes. Taste for seasoning and add another pinch or two of salt if desired.
Put half of the mixture in a blender with 1 cup of the chicken stock and half the drained chiles. Blend until smooth, about 3 minutes on high. Repeat with the remaining sauce mixture, another 1 cup of chicken stock, and the remaining chiles.