For the spice rub: Toast the ancho chile pieces over low heat in a dry skillet until fragrant, shaking the pan so they don't scorch. Put the chiles in spice grinder and pulse to a powder. Add the remaining ingredients and buzz again to combine. (Reserve some to sprinkle on the corn.)
For the pork: Rub the spice mix into the pork and set aside for 1 hour while you prepare the grill. (You can marinate the pork longer under refrigeration, just make sure to bring it to room temperature before cooking it.) Light the coals in a barbecue grill with a cover. When the coals are hot, push them to 1 side of the grill. Rub the pork with some olive oil and season it liberally with salt. Put the pork on the grill away from the coals and place a drip pan underneath the meat. Close the cover and cook, maintaining a medium low temperature, for about 4 to 6 hours, or until the meat shreds easily. Turn the meat occasionally and baste with the drippings.
For the drizzling sauce: Combine all ingredients in a blender and puree until smooth; taste and season with salt and pepper. Drizzle over the shredded pork.