Using paper towel, pat chicken dry all over and inside the cavity. Separate the skin from the meat by gently sliding you fingers under the breast skin and in and around the thigh and leg, taking care not to break the skin. Place chicken on an ovenproof wire rack in the fridge, uncovered, for the skin to dry. Stand for at least 1 hour or if time permits, overnight.
Meanwhile, for the eggplant butter, place the pickled eggplant and ¼ cup water in a small saucepan over medium-high heat and stir until melted and smooth. Cool Completely.
In the bowl of a stand mixer fitted with the paddle attachment, beat butter on high speed for 6-8 minutes until pale and fluffy. Reduce speed to low and add the remaining ingredients and half the pickled eggplant mixture. Season to taste and mix until well combined. Roll mixture into a log and wrap in plastic wrap. Chill until set, then slice half the butter into 5mm-thick slices. Return sliced butter to the fridge. Place remaining butter in a small bowl at room temperature and set aside.
To make the apricot couscous stuffing, place the couscous and lemon zest in a heatproof bowl and stir in the hot stock. Cover tightly with plastic wrap and stand for 6-8 mins until couscous is tender. Fluff with a fork. Add remaining ingredients and remaining pickled eggplant mixture, season to taste and stir to combine. Set aside until ready to use.
Preheat oven to 200°C. Spread sliced butter under the skin of the chicken, making sure it's under the breast skin, leg skin and thigh skin. Place as much couscous mixture as you can into the chicken cavity, then tie the legs together with kitchen string. Season chicken all over and reserve remaining couscous mixture. Place chicken on a wire rack in a deep roasting pan and roast for 1 hour - 1 hour 15mins until the chicken is cooked through. Remove from the oven and rest, loosely covered with foil, for 15-10 mints.
place chicken on a platter and served with reserved softened butter and remaining couscous mixture alongside.