Roast the poblano chile by placing it directly on a gas stove flame, or under an electric broiler.
Once the skin is well blackened all over, wrap the roasted chile in a clean dish towel, then place in a paper bag and close tightly so the chile can sweat for about 15 minutes.
Meanwhile, in a 2-quart saucepan, melt butter.
Add the onion and sauté for about 3 minutes.
Incorporate the flour by sprinkling it in the saucepan and whisking well.
Once all of the flour has been absorbed and moistened, add the milk and chicken stock. Stir and continue to heat.
Remove the chile from the bag and towel, and scrape well to remove the blackened skin.
Rinse under running water, then slice open and remove the seeds and the stem.
Slice the chile into strips, then dice into 1/2-inch squares. Add to the soup, along with the fresh corn kernels.
Heat for another 5 minutes, and season with salt and pepper. Serve immediately.