Peel the outer skins off the tomatillos, onion and garlic. Over an open flame on your stovetop or grill on your BBQ the tomatillos and peppers until charred all over.
Remove from the flame or grill and place the tomatillos aside and the peppers in a brown bag. Seal and let steam for at least 10 minutes to remove the charred skins more easily. With rubber gloves remove the skins and seeds from the peppers and discard.
Place all the peppers, tomatillos, garlic, onion and salt in a food processor, blender and pulse until smooth.
Heat the oil in a large stock pot and pour in the puréed chile mixture. Add the chopped potatoes, stir and cover. Simmer until potatoes are fork tender, about 15 minutes. Slightly mash in the pot to thicken the soup and add the chicken stock. Stir to mix and add the shredded chicken. Cook another 10 minutes.
For the optional mango sauce, peel and seed the mangoes, toss them in a blender and add the Sriracha sauce. Purée until smooth. Set aside.
Season to taste with salt. Ladle into bowls and drizzle with Mexican Crema and Mango Sriracha Sauce (optional). Top with chopped cilantro and serve.