Combine pasilla sauce and chicken in a large skillet. Cook uncovered over medium heat approx. 10 minutes or until heated through, stirring occasionally.
Brush tortillas with butter on one side. Place 4 tortillas buttered side down on a baking sheet.
Spoon 1/4 of the chicken mixture on the center of each prepared tortilla. Sprinkle 1/4 cup of cheddar and 1/4 cup monterey jack over the chicken on each tortilla. Cover each with the remaining tortillas, butter side up.
Bake at 375 degrees for 9-11 minutes until tortillas are crisp and golden brown.
Cut into wedges and serve with sour cream, guacamole and/or salsa.