In a large bowl, mix cilantro, minced garlic, salt, black pepper, orange juice, olive oil, and annatto.Let stand while you cut the chicken.
2. Cutting the chicken
With a sharp knife, butterfly cut the chicken, dividing each breast into two halves as separate fillets.
3. Marinate the chicken
Add the chicken breasts to the marinade, and stir to coat them all in the marinade. Cover with a lid or plastic wrap and refrigerate for 2-3 hours.
4. Cooking the chicken
Heat griddle, grill or skillet over high heat. Add, depending on the size of the pan or grill, two or three fillets and cook for 3 minutes or until lightly browned.Turn over, and cook the other side, or until the internal temperature reaches 74 ºC [165 ºF] measured with a meat thermometer (about 6 minutes in total).
5. Making the sauce
While the breasts are cooking, heat the marinade over medium heat in a small saucepan until reduced by half. Remove from heat.
6. Serve
Serve the breasts accompanied by the sauce in a small bowl.