How to Make It
Roast the Anaheim and poblano chiles over a gas flame or under a broiler, turning often, until charred all over. Put the chiles in a bowl, cover with plastic wrap, and let steam for 5 minutes. Peel the charred skin, allowing the juices to fall into the bowl. Discard the cores, ribs and seeds, coarsely chop the chiles and transfer to a small bowl. Strain the chile juices over the chopped chiles.
Heat the olive oil in a medium nonreactive saucepan. Add 1/2 cup of the corn kernels and the chopped onion and cook over moderate heat, stirring occasionally, until the onion is wilted, about 4 minutes. Add the chicken stock to the pan and raise the heat to high. Boil until the liquid is reduced by half, about 5 minutes. Stir in the roasted chiles with their juices and the cilantro and season with salt and pepper.
Transfer the mixture to a blender or food processor and puree until smooth. Return the sauce to the saucepan and add the remaining 1/4 cup corn kernels. Bring the corn sauce to a simmer over low heat and whisk in the butter until incorporated; do not let the sauce boil. Remove from the heat and season with salt and pepper.