1. In a deep pan, cover the pork with water, add sea salt and cook covered over medium heat until tender but not soft - about 40 minutes. Drain, reserving the broth. Cool and skim, reserving the fat.
2. In an ungreased, frying pan, toast the sesame seeds until a deep gold colour, taking care not to burn them. Cool and then grind to a textured powder in an electric coffee grinder.
3. Add vegetable oil to the frying pan, flatten out the Ancho chile, and fry for a few seconds until the inside turns a light brown. Drain, tear into pieces, and put into the blender jar. In the same fat, fry the onion, garlic, chiles and whole tomatoes for a bout 1 minute or until the onion is translucent. Transfer to the blender jar. Add the remaining oil to the pan and fry the bread until crisp on the outside; crumble into the blender jar. Add 250ml of the reserved broth and blend until fairly smooth.
4. Put 2 tablespoons of fat from the broth into a flameproof casserole, add the blended ingredients and the ground seeds, and fry over medium heat, stirring and scraping the bottom of the pan to avoid sticking, for about 5 minutes. Add the meat and the rest of the ingredients (whole), along with the remaining broth and cook over low heat for 10 more minutes. Adjust seasoning. The sauce should be of medium consistency and thinly cover the back of a wooden spoon.

(Reproduced with kind permission of Diana Kennedy from "The Essentials of Mexico", which is published by Clarkson Potter/Publishers)