Remove the stems, if any, from the chiles, slit them open, and remove veins and seeds. Toast the chiles on a hot comal for a few seconds on each side, pressing them down until the inside flesh turns an opaque, tobacco color. Rinse the chiles in cold water, cover with hot water, and set aside to soak for about 15 minutes. Put the tomatoes into a blender jar and blend briefly. Heat 1 tablespoon of the lard and fry the sesame seeds for a few seconds until a deep golden brown. Transfer with a slotted spoon, draining them as much as possible, to the blender jar; add the oregano, cloves and allspice and blend until smooth, adding a little more of the water in which the chiles were soaking if necessary. Add more lard to the pan and heat; add the onion and garlic and fry until translucent. Add the cinnamon pieces and fry until the onions and garlic are lightly browned. Transfer with a slotted spoon to the blender jar. Add the plantain and bread to the pan and fry over low heat until a deep golden color; transfer to the blender jar. Adding more chile water if necessary, blend until you have a smooth puree. Gradually add the soaked chiles with more water as necessary and blend until smooth. When all the chiles have been blended, dip a spoon into the bottom of the blender jar and take out a sample of the sauce to see if the rather tough chile skins have been blended sufficiently. If not, add a little more water, stir well, and then blend for a few seconds more. Heat the remaining lard in a heavy pan or saute pan, add the blended sauce and chocolate, and cook over medium heat, stirring and scraping the bottom of the pan to prevent sticking, for about 15 minutes.
The sauce should be fairly thick and lightly cover the back of a wooden spoon.