Char the peppers over the stove burner flame until blackened all over. Place in a large bowl and cover tightly with a dish towel. Let cool. Peel and scrape off the blackened skins. Remove the stems and seeds from the peppers and cut into 1/4-inch pieces.
Melt the butter in a medium Dutch oven over medium heat. When the butter is melted, add the leeks, potatoes and peppers and cook until the leeks are wilted, about 4 minutes. Add the garlic and season with 1 teaspoon salt. Sprinkle with the flour and cook to remove the raw flour taste, about 2 minutes. Add the cumin, oregano and nutmeg. Pour in the chicken broth and 2 cups water. Stir to combine. Bring to a strong simmer and cook until the potatoes are tender, about 15 minutes.
Add the cream, chicken and corn. Return to a simmer and cook until slightly thickened and flavorful, 10 to 15 minutes more. Stir in the lime juice and season with salt if needed.
Serve the soup in bowls garnished with cilantro leaves and cotija.