There appear to be a couple of versions of this Oaxacan paste, the version I made from the Roberto Santibanez book is a paste of pumpkin seed and dried chile, however looking on-line there are also versions of the sauce which include dried shimp as part of the paste.
The procedure is simple and consists of the following blended together (in a food processor as the paste is too thick to be made in a blender):
3/4 of a cup of toasted pumpkin seeds
About 10 toasted dried chiles
A tablespoons of cider vinegar and another of rice wine vinegar
Enough olive oil to form the paste
I tried this a number of times using both pasilla and guajillo chiles and also using chiles which had been soaked in water after toasting. The recipe in the book calls for Passilla de Oaxaca chiles which are hotter and smokier than regular passila chiles, but which I have never been able to find in the shops. The paste is quite thick and I think requires more than the tablespoon of oil that Santibanez suggests to achieve the right consistency.
The paste works well with chicken and is also a good addition to the tortilla for a quesadilla, though it benefits from a days resting for the flavours to mellow.