4 dried pasilla chiles, about 1 ounce total
1/4 cup white wine vinegar
2 medium garlic cloves, crushed
2 tablespoons granulated sugar
1/2 teaspoon ground cumin
1/2 teaspoon dried oregano
Freshly ground black pepper
2 pork tenderloins, about 1 pound each
1/4 cup ketchup
1/2 cup heavy cream
Remove the stems and seeds from the chiles. Cut the chiles into sections about 2 inches long. In a medium skillet warm 2 tablespoons of canola oil over high heat. Add the chiles carefully (the oil might splatter) and toast them until fragrant, 3 to 5 seconds per side.
Transfer the chiles and oil to a small bowl. Cover with 1-1/2 cups of hot water. Use a smaller bowl or plate to submerge the chiles. Allow to rehydrate for 20 minutes.
Pour the mixture, chiles and water and all, into a blender or food processor. Add the vinegar, garlic, sugar, cumin, oregano, 1/2 teaspoon of salt and 1/4 teaspoon of pepper. Process until smooth. Press the mixture through a medium-mesh strainer set over a bowl. Discard the solids left in the strainer.
Trim the pork tenderloins of any excess fat and sinew. Place them in a large, plastic resealable bag and pour in 1 cup of the chile mixture, reserving the rest of the mixture. Press the air out of the bag and seal tightly. Turn the bag to distribute the marinade, place the bag in a bowl and refrigerate for 6 to 8 hours, turning the bag occasionally.
Pour the reserved chile mixture into a medium saucepan. Whisk in the ketchup and cream. Bring the mixture to a simmer and cook until it reaches a sauce consistency, 3 to 5 minutes. Season with salt and pepper to taste. Set the saucepan aside.
Remove the pork tenderloins from the bag, discarding the marinade. Lightly brush or spray the tenderloins all over with canola oil. Season generously with salt and pepper. Grill the tenderloins over Indirect Medium heat until the centers are barely pink and the internal temperature reaches 155°F, 25 to 30 minutes, turning once halfway through grilling time.
Remove from the grill, loosely cover with foil, and allow to rest for 3 to 5 minutes before cutting into slices. Reheat the sauce over medium heat and spoon it over the meat.