Use kitchen tongs and hold peppers directly above the gas flame. Cook until blackened; 4-6 minutes. Place peppers in a bowl and cover tightly with plastic wrap. Let chilies steam for 10 minutes. OR - Roast the Poblano chilies under the broiler, turning often, until charred.
Using plastic wrap or gloves, rub the skin of the peppers to remove as much of the blackened skin as you can.
Remove and discard stems and seeds. Finely dice one of the peppers, cut the other into strips, and set aside, separately.
Melt 1 Tbsp of butter in a medium skillet. Add onion and pepper strips. Cook 4 minutes over medium high heat, until onion is soft.
Warm vegetable stock in a saucepan on low.
Combine onion/pepper mixture and one-half of the warmed stock in a blender and process until smooth; about 30 seconds.
Add the other 2 Tbsp of butter in the same skillet over medium heat. Whisk in flour, and reduce heat to low. Cook, whisking constantly, until mixture is golden brown. (3 minutes.)
Increase heat to medium. Add onion/pepper purée and remaining 1C warmed stock, and cook, whisking constantly, until thick enough to coat the back of a spoon.
Reduce heat to low, add buttermilk and diced chili. Simmer gently to let flavors meld; about 2 minutes.