Slice the peppers in half lengthwise; remove stem, seeds and membrane. Place under the broiler skin side up until the skin turns black and blisters. Remove and immediately place in a small paper bag or plastic container with a tight lid. Place in the freezer for 10 minutes; this will loosen the roasted skin. Remove peppers and rub off the black skin with your fingers. Chop skinned peppers and set aside.
Saute the butter, onions, and garlic until soft in a heavy bottomed soup pot on medium-low heat. Add the reserved red peppers, tomato paste, and chicken stock. Bring to a low boil, reduce the heat, and then simmer for 10 minutes. Stir in the heavy cream, cilantro, salt, and pepper. Stir in the cornstarch mixed with a little cold water to thicken. Simmer 5 minutes. *Let the soup cool slightly, and then blend in batches in a blender, or use an immersion blender to puree. Sprinkle the roasted corn over the soup before serving and top with a dollop of Cilantro Lime Sour Cream.
* When blending hot liquids: Remove liquid from the heat and allow to cool for at least 5 minutes. Transfer liquid to a blender or food processor and fill it no more than halfway. If using a blender, release one corner of the lid. This prevents the vacuum effect that creates heat explosions. Place a towel over the top of the machine, pulse a few times then process on high speed until smooth.
Cilantro-Lime Sour Cream:
1 cup sour cream
1/2 lime, juiced
1 teaspoon finely minced fresh cilantro leaves
In a mixing bowl, whisk all ingredients vigorously until light and creamy.