In a large pot, place the chicken, quartered onion, garlic, oregano, thyme, cilantro, and peppercorns. Cover with water by 2 inches and bring to a boil. Lower the heat and simmer until the chicken is just cooked through, 20 to 30 minutes, occasionally skimming any foam that rises to the surface. Remove the chicken and set aside. Strain the stock through a fine mesh strainer and reserve for the mole.
In a large heavy dry skillet, toast the chiles to a deep brown color over medium-high heat, turning, 1 to 2 minutes. Place in a bowl, cover with hot stock, and soak for 20 to 30 minutes. Drain and transfer to a blender. Puree, adding chicken stock as necessary, to blend.
In a large skillet or saute pan, heat 1 tablespoon of the oil over medium-high heat. Add the sesame seeds and peanuts, and cook, stirring, until golden brown, 2 to 3 minutes. Drain on paper towels and add to the blender with the chiles. Heat 1 tablespoon of the oil in the pan over medium heat. Add the onions and garlic and cook, stirring, until soft and starting to color, 6 to 7 minutes. Add the cinnamon, black peppercorns, cloves, and coriander seeds and cook, stirring, until golden brown, about 2 minutes. Add to the blender. Increase the heat to medium-high, add the tortilla pieces, and cook until golden brown, 1 to 2 minutes. Add to the blender with enough of the chile soaking liquid (or chicken stock) to puree to a paste.
To the skillet, heat the remaining 3 tablespoons oil over medium heat. Add the paste and and simmer, stirring constantly, for 5 minutes. Add 2 cups of chicken stock, the chocolate, and salt, and bring to a boil. Reduce the heat, and cook, stirring, to melt the chocolate. Add the cooked chicken and simmer, stirring occasionally, for 20 minutes, adding more stock, as needed, if the sauce becomes too thick, and basting the chicken occasionally as it cooks. Remove from the heat and adjust the seasoning, to taste.
To serve, spoon the rice into the center of a large platter. Arrange the chicken and sauce over the top, garnish with chopped cilantro, and serve with hot corn tortillas.