Pasilla chiles are dried chiles with a rich flavor. They vary in heat from mild to medium-hot. For best results, use kitchen shears to remove the stem, then cut along one side of chile to remove the seeds and veins. Look for them in Latin grocery stores.
Pasilla sauce can be made ahead of time. Store in airtight container in refrigerator up to 3 days, or in freezer up to 1 month.
For a more traditional version, substitute thin beef steak strips for the pork.