Heat oven to 400° F., Combine the enchilada sauce, chocolate, and raisins in a microwave-safe bowl; microwave until the chocolate is melted, 1 to 2 minutes. Transfer to a blender, add ¼ cup water, and process until very smooth.
Heat 1 tablespoon of the oil in a large ovenproof skillet over medium-high heat. Season the chicken with ¼ teaspoon each salt and pepper and cook until golden brown on one side, 3 to 4 minutes.
Add the mole sauce and turn the chicken to coat. Transfer to oven and roast, browned-side up, until cooked through, 12 to 14 minutes.
Meanwhile, combine the vinegar, the remaining 2 tablespoons of oil, and ¼ teaspoon each salt and pepper in a large bowl, Add the watercress, orange, and onion and toss to combine.
Serve the salad with the chicken and mole sauce and sprinkle with the sesame seeds.