Step 1) Combine Spice Blend ingredients in a small bowl; mix well.
Step 2) Place 3 tablespoons of Spice Blend,in a large resealable plastic bag. Add chicken, seal bag and turn to coat chicken thoroughly with spices. Marinate in refrigerator at least 15 minutes.
Step 3) Preheat grill to medium heat or about 350°F.
Step 4) Grill seasoned chicken for 4 to 6 minutes per side or until cooked through.
Step 5) To make Ancho Chile Sauce: Heat oils in a small saucepan over medium heat. Add onions and cook until softened.
Step 6) Stir in remaining Spice Blend and tomato sauce, stirring well. Heat 1 minute, then remove from heat. Stir in chocolate until melted and set aside to cool slightly. Serve immediately, or cover and refrigerate up to 3 days.
Step 7) Garnish with sesame seeds, fresh lime wedges, or cilantro, if desired.