Roast corn and peppers over grill or under broiler until slightly charred. Put peppers in bag to steam for 5 minutes then peel. Leave bits of charred skin on, if desired. When cool enough to handle, cut kernels from corn cobs, reserving cobs. While vegetables are roasting, heat olive oil in Dutch oven and sauted onion and garlic until tender. Add chicken broth. Add corn cobs and simmer 15 minutes. Add potato and simmer another 10 minutes. Remove cobs. If desired, scrape cobs over soup to extract corn remaining on cob. Discard cobs. Dice peppers. Add peppers, corn kernels, milk and red pepper flakes, if desired. Simmer another 10 minutes. Add cilantro and season to taste with salt and pepper.