Cut out and discard the stems of the chard, then wash and finely chop the leaves to measure about 6 cups. set aside.
In a large pot, heat 2 T. of the stock over med. heat. when it is steaming, add the onions and garlic and stir and saute for 3 mins.
Add the rem. 8 cups of stock, the tomatoes, potatoes, chili powder, spices and simmer over med. high heat.
Reduce the heat to med. low and simmer for 10 mins.
Add the chard and corn, return to a simmer, and cook about 10 mins or longer, until the soup has been reduced to a nice thick consistency.
Serve hot, garnished with cilantro, if desired.