Preheat oven to 375 degrees F. Lightly coat a 2-quart rectangular baking dish with cooking spray; set aside.
For sauce, in a small saucepan heat butter over medium heat until melted. Add onion and garlic; cook and stir about 3 minutes or until very tender. Add tomatoes, honey, vinegar, ground ancho pepper, salt, and cumin. Bring to boiling; reduce heat. Simmer, uncovered, for 5 minutes. Stir in chocolate and snipped cilantro until chocolate is melted. Cool slightly.
Place chicken in the prepared baking dish. Spoon half of the sauce over chicken; turn chicken to coat. Bake for 30 to 35 minutes or until chicken is no longer pink (170 degrees F). Reheat the remaining sauce until warm. Serve chicken with warm sauce. If desired, garnish with cilantro sprigs.