To make the mole sauce, soak the chiles in 1 1/4 cups water for 15 minutes. Drain well and discard the soaking liquid.
Heat the oil in a large heavy saucepan over medium heat. Add the onions and garlic and saute until translucent, 5 minutes. Transfer to a blender with the chiles, chocolate, chicken broth, peanut butter, sugar, oregano and tostadas and blend until very smooth. Transfer the sauce to a saucepan and bring to a boil over high heat. Reduce the heat to medium, cover and simmer for 20 minutes.
Season with salt and pepper. Add the rotisserie chicken pieces and stir to combine.