Take chicken from fridge 2 hours before cooking and pat dry. Cut off the neck and wing tips and remove the wishbone. Turn the chicken around and remove the parson’s nose. Stand chicken up with its bottom facing you and, using a large sharp knife or kitchen scissors, cut down each side of the backbone and remove it. Flatten the chicken out on the board and season the skin with salt.
Preheat the barbecue to medium. Rub the chicken with oil and place skin side down on barbecue. Cook for about 15 minutes until skin is browning. Turn chicken over and cook for another 15 minutes. Reduce heat to lowest setting and place lid on barbecue, or cover with an upturned roasting tray. Cook for a further 15 minutes approximately, until the juices run clear. Remove from the barbecue, cover with foil and two tea towels to help retain heat, and rest for about 15 minutes.
Cook the corn on the barbecue for 10 minutes until charred and softened.
For the dressing, place all the ingredients in a bowl and gently mix together until combined. Check seasoning.
Place chicken on a chopping board and cut off the legs, then cut each leg in half at the joint where the thigh and drumstick meet. Cut the breast in half down the middle, where it looks like the meat distribution will be even. (This means the thicker part of the breast will be shorter in length to get the same amount of meat.)
Cut the grilled kernels off the corn cobs. Place in a bowl with chopped cos leaves, diced avocado and cherry tomato halves. Drizzle with ranch dressing and serve with the chicken.