Over gas flame or on barbecue grill roast chiles until charred on all sides. Place in a paper bag for 20 minutes. Peel off blackened skin, remove seeds and membranes and chop into coarse pieces.
In a large heavy stew pot heat bacon over high heat and saute until 3 tablespoons of fat are rendered. Reserve bacon for another use or discard. Add onion and garlic and saute until onion browns. In a large bowl mix together ancho chile molido, salt and masa harina. Add pork and toss to coat in spice mix. Push onion to sides of pot and add seasoned pork cubes, turning several times until well-browned. Stir together pork and onions, add stock, scraping up any browned bits from bottom of pan, and bring to a boil. Reduce heat to simmering and add roasted poblano. Stir well, cover and cook about 2 hours, or until pork is very tender. If using cumin, add 5 minutes before end of cooking. Taste and season with salt and pepper if needed. Serve with warm corn tortillas and garnish as desired with cumin, posole, lime juice, cilantro, or extra chopped roasted chiles.