1. Place the Pasilla chilies over a high flame, turning them as they blacken.
2. Cover the chilies with a clean towel and let them sweat for 10 minutes.
3. Remove the chilies from the bag and peel off the charred skin with a paring knife or your fingers. Remove excess pieces of skin with a paper towel. Rinse the chilies in cool water to remove any remaining skin bits or seeds.
4. Make a slit in each chilie and remove the seeds.