Preheat the oven to 190°C. Combine all the harissa butter ingredients in a food processor and blend until almost smooth. Position the fat chicken onto a clean, flat surface and wiggle its legs; now push your fingers gently under the skin on the neck end and carefully work the skin away from the flesh. If you have short fingers, use the handle of a wooden spoon and gently lift the skin, without poking holes in it. Once you have lifted the skin off the breast, gently work your fingers down to the thighs and lift the skin there. Divide the harissa butter and push half of the butter under the skin. Position it onto the breast bone and then stroke it down to the thighs, and make sure you cover the breast as well.
Rub your buttery hands all over the top and bottom of the chicken and season with salt and pepper. Roast the chicken in the oven until is golden and crisp. Roast it for 15-20 minutes for every 400g. The legs should wiggle when you move them and the juice run clear from the thickest part of the chicken. Please take the time to rest your chicken before you start to carve it. You will be rewarded with a juicy, succulent meal with all the flavour on your plate and not on the chopping board. Serve with a roasted aubergine and courgette salad.