Heat the oven to broil with the rack 4 to 6 inches from the heat element. Lightly grease a large sheet pan; set aside.
In a small bowl, stir together 2 tablespoons of the harissa and the yogurt. Rub the yogurt mixture over the chicken; set aside.
In a medium bowl, whisk together 1 tablespoon olive oil, remaining 1 tablespoon harissa and salt. Add the broccolini and toss to coat.
Arrange the chicken and broccolini in a tight single layer on the prepared pan. Broil 10 to 12 minutes, turning the chicken halfway through until chicken is fully cooked and no linger pink inside and the broccolini is deep golden brown. (if your chicken breasts are especially large, you may need to cook them longer)
Meanwhile, prepare the couscous according to package directions. Serve chicken and broccolini with couscous and parsley, if desired.