Toast the pumpkin seeds in an ungreased pan, turning them over and shaking the pan from time to time to prevent them from burning. Set them aside to cool and then grind them in an electric grinder to a fine powder. Put 1 1/2 cups of the broth in a blender, gradually adding the tomates verdes, lengua de vaca, hoja santa, chopped epazote, and chiles, blending as smooth as possible.
Heat the lard in a flameproof casserole in which you are going to serve the mole and fry the blended ingredients, stirring and scraping the bottom of the pan to prevent sticking; cook for about 25 minutes. Add the rest of the epazote.
Stir 1/2 cup of the broth into the pumpkin seeds until you have a smooth consistency and gradually stir into the cooked ingredients. Continue cooking over low heat, stirring constantly for 10 more minutes. Take care that the pumpkin seed mixture does not form into lumps; if this happens, put the sauce back into the blender and blend until smooth. Add any remaining broth and salt, to taste. If the sauce is too thick, add a little water to dilute.
Recipe excerpted from My Mexico by Diana Kennedy: Clarkson N. Potter, Publishers, 1998