With a sharp, heavy knife, cut each chicken breast in half crosswise for a total of 6 pieces. In a large glass or plastic bowl, combine cilantro, tequila, lime juice, brown sugar, chile, garlic, salt, and pepper. Add chicken and mix to coat with marinade. Cover and let stand at room temperature for 30 minutes. (Do not marinate longer, because the acid in the marinade will begin to “cook” the chicken.).
Prepare a gas or charcoal grill for direct, medium-high heat (you can hold your hand 1 to 2 inches above the cooking grate only 2 to 3 seconds; visit our Grilling Essential section for complete instructions).
While grill heats, remove chicken from marinade, transfer to a platter, and pour marinade into a small bowl. Push your fingers under skin of each piece of chicken and rub meat with 1/2 teaspoons lime zest.
Place chicken skin side down on oiled cooking grate and brush with marinade; cover gas grill. Cook until skin is browned but not charred, 4 to 6 minutes. Turn chicken with a wide spatula, brush again with remaining marinade, and cook, turning as needed to prevent burning, until chicken is no longer pink at the bone (cut to test), about 10 minutes longer. Transfer to a clean platter.