In a small bowl or shallow dish, whisk together the olive oil, lemon juice, brown sugar, harissa paste, minced garlic, salt, smoked paprika, cumin, coriander and cayenne, if using it. Add the chicken thighs and turn them to coat each piece thoroughly. Refrigerate the chicken in the marinade, covered, for eight hours or overnight.
Drain and discard the marinade. Grill the chicken, covered, on an oiled rack over medium heat for six to eight minutes on each side or until a thermometer inserted into the thickest part of the chicken reads 170°F.
During the last five minutes of cooking, baste the chicken occasionally with honey.
Remove the chicken to a cutting board and let it rest for five minutes before slicing it into ½-inch pieces.
