How to Make It
Using kitchen tongs, hold 1 chile directly over a medium flame of a gas stovetop. Cook until skin is blackened, 4 to 6 minutes per side. Repeat with remaining chile. Place chiles in a bowl, cover with plastic wrap, and let steam 10 minutes.
Using same piece of plastic wrap, partially cover your hands, and rub off skin from chiles, removing as much of the blackened skin as you can. (Don’t worry if all of the skin doesn’t come off.) Remove and discard stems and seeds. Finely dice 1 chile; cut remaining chile into strips, and set aside separately.
Melt 1 tablespoon butter in a medium skillet over medium-high. Add onion and chile strips. Cook until onion is soft, about 4 minutes. Combine onion mixture and 1/2 cup warm stock in a blender, and process until smooth, about 30 seconds.
elt remaining 2 tablespoons butter in same skillet over medium. Whisk in flour, and reduce heat to low. Cook, whisking constantly, until mixture is golden brown, about 3 minutes. Increase heat to medium. Add onion-chile puree and remaining 1 cup warm stock, and cook, whisking constantly, until thick enough to coat the back of a spoon, about 2 minutes. Reduce heat to low; add buttermilk and diced chile. Simmer gently to allow flavors to meld, about 2 minutes. Season with salt and white pepper.
You can roast poblanos under broiler, turning often, until charred.