Preheat the broiler and line a baking sheet with foil.

Place the chiles in a single layer and place in the top third of the oven. Roast, turning the chiles every 3-5 minutes, until the peppers are blistered and blackened, about 15 minutes.

Remove the baking sheet from the oven and use tongs to transfer the chiles to a large bowl. Cover immediately with plastic wrap or place a plate over the bowl and the let the chiles sit and steam for 15 minutes.

Once the chiles are cool enough to handle, peel the blackened skin from the chiles. Remove the stems and the seeds and place the chiles in a blender or food processor.

To the blender, add 2 cups of the buttermilk and the flour. Process until smooth.

Pour the chile mixture into a large skillet or a saucepan. Add the remaining buttermilk and season to taste with salt. Cook over medium-low heat until it is warmed through and the flavors are combined.

Stir in the sour cream.

You can use the sauce right away or transfer to the refrigerator until ready to use.