In a medium size heavy saucepan, heat the olive oil over moderate heat for 1 minute; add the onion and garlic and cook, uncovered, until soft about 5 minutes.
Add the tomatoes, vinegar, molasses, mustard, chili powder, and cayenne pepper; simmer, uncovered, for 20 minutes or until the mixture is slightly thickened.
Cool about 20 minutes, then whirl for 30 seconds in an electric blender or food processor.
Pour the sauce into a large, shallow enameled or glass bowl, add the chicken thighs, and turn well to coat.
Cover and refrigerate at least 4 hours, turning the thighs occasionally.
Preheat the broiler and lightly grease the rack.
Broil the chicken 7 to 9 inches from the heat for 15 minutes, basting occasionally with the sauce.
Turn and broil 15 minutes longer, basting occasionally, until browned.