To brine the chicken, stir together salt, cinnamon, rosemary, the 1 cup of apple cider vinegar and honey in a container large enough to hold the chicken and brine. Pour in boiling water and stir to dissolve.
Add ice, stir until melted and water is chilled, then submerge the chicken, placing a plate on top to prevent it floating. Cover with clingfilm and refrigerate for one to three hours, but no longer.
Remove chicken from brine and thoroughly pat dry with paper towels. For best results and for a super-crispy skin, place in the refrigerator to air-dry, uncovered, for at least another hour, or even overnight.
Remove chicken from the refrigerator and let it stand at room temperature for 30 minutes.
Preheat oven to 200°C. Blanch leeks in a pot of boiling salted water for 3 minutes. Drain well, patting down with paper towel. Arrange in a roasting tray with 2 tbsp olive oil, and seasoning.
Rub chicken with 1 tbsp olive oil, then liberally season inside and out.
Place chicken on a rack set inside a roasting pan. Roast for about one hour, until thigh juices run clear, or, if you have a thermometer, until the temperature reaches 70°C at the thickest part of the thigh (being careful not to touch the bone, as this will give you a false read).
For the last 10 minutes, add in the leeks and roast until golden and caramelised.
Cover chicken with foil and rest in a warm place for 10 to 15 minutes.
Make a dressing from the remaining 2 tbsp olive oil, the 2 tbsp apple cider vinegar and seasoning. Pour this over the leeks as soon as they come out of the oven.
Carve the chicken and serve with the leeks, garnished with hazelnuts and chives.