Sopa de Crema de Poblano y Papas (Cream of Poblano and Potato Soup)

4 lg poblano chiles
1 tbsp unsalted butter
1 tbsp vegetable oil
1 medium sweet onion, grated
2 lg garlic cloves, crushed
3 yukon gold potatoes, peeled and thinly sliced
2/3 cup water
1/4 cup cilantro, plus more for garnish
1 cup chicken stock**
1/2 cup frozen corn kernels (thawed) 
1/2 cup heavy cream
1/2 teaspoon salt

** I recommend a stock simmered with chiles.   My recipe is in my blog

1. Roast the chiles directly over a gas flame or under a preheated broiler, turning frequently with tongs, until charred all over, about 3 to 4 minutes.  Watch carefully to prevent charring too much or the chiles may get too soft.  Place in a plastic bag and steam about 5 minutes.  Scrape off the charred skin.  Cut the chiles open, remove the seeds and veins.  Chop coarsely and reserve.

2. In a large heavy saucepan, heat the butter and oil over medium heat.  Cook the onion stirring frequently until softened, 3 to 4 minutes.  Add the potato, water, garlic cloves and chicken stock.  Bring to a boil then reduce the heat to medium-low and cook until the potato is very tender, about 15 minutes.

3. When the potato is done, either use a stick blender to blend the contents of the saucepan, or use a blender or food processor.  Add the reserved chiles, cilantro cream and salt.  Puree until smooth.  Return the soup to the saucepan, and bring to a boil over medium-low heat, stirring frequently to prevent scorching.  Divide the soup among four bowls.  Garnish with about 2 tablespoons of corn, and sprinkle with cilantro