Heat a grill to medium (350° to 450°). Whirl peppers, cashews, vinegar, lemon juice, hot sauce, and salt in a blender. With motor running, add 1/3 cup oil in a steady stream through opening in lid until sauce is smooth and lighter in color. Set aside half the sauce.
Put chicken in a bowl and drizzle with remaining oil. Lay chicken skin side down on cooking grate and brush with some of remaining romesco sauce. Grill, turning often and brushing each time with more sauce, until chicken is well browned and cooked through, about 20 minutes. Serve over couscous with reserved sauce on the side.