Preheat the oven to 300 degrees F.
In a small skillet, combine the coriander seeds and peppercorns. Cook over medium heat, stirring frequently, until fragrant and toasted, about 4 minutes. Remove from the heat and let cool. In a coffee grinder, process the spices until finely ground.
In a small bowl, combine the peppercorn mixture, salt, ancho chile pepper, cinnamon and cocoa. Rub the spice mixture evenly over the pork roast.
In a Dutch oven, combine the beef stock and onions. Add the roast to the Dutch oven. Cover and bake until very tender, about 3 hours.
Using two forks, shred the roast. Serve on buns with pickle spears if desired.