Set the broiler on high, lay both the Poblano and Jalapeno peppers in one layer on a baking tray and broil until the skin is black and blistered; turning as needed to roast on all sides. Remove the jalapeno early if necessary as the Poblano will take longer.
Transfer the charred peppers to a plastic bag, tie the top closed and let steam until cool to the touch, about 15 minutes.
Peel the skins off and discard (they should just slip off) and remove the stems and seeds. Cut into dice. Set side.
Put the milk into a large pan. Add the chipotle pepper, bay leaf and rosemary to the milk. Bring the milk to a simmer, cover and turn off the heat and let infuse for 20 minutes.
In a large soup pan, melt the butter and oil over medium heat. Add the onion and salt and cook until softened, but not browned.
Reduce the heat to medium-low and add the garlic, flour and ground cumin and cook for another minute. Add the corn and roasted poblanos and cook for a few minutes.
Strain the milk through a sieve and add it to the vegetables. Bring to a simmer and cook for about 15 - 20 minutes (do not boil). Remove from the heat and leave to cool for a few minutes.
Put half the soup into a blender and blend until smooth and add it back to the pan.
Check seasoning.
Serve in deep bowls and top with a dollop of sour cream, crispy tortilla strips and cilantro.