Begin by spatchcocking the chicken. On a chopping board, use a sharp knife to make a few deep
slashes through the meat, on both the breast and legs (this helps the heat penetrate to the middle more quickly). Turn the bird breast-side down on the chopping board and, using heavy-duty kitchen scissors, cut down each side of the backbone. Remove and discard. Turn the bird breast-side up and press down firmly to open it out flat.
Fire up the barbecue ready for direct and indirect cooking. In other words, have hot coals on one side and none on the other, reserving a space away from the heat source (it'll still get warm and will allow for a slower, more even cook).
Drizzle the chicken with the olive oil and season well with salt and freshly ground black pepper. Lay onto the grill bars, away from the heat source, skin-side up. Rest a heavy object on top to weigh it down, like a heavy frying pan. Shut the lid. Leave to cook for 45 minutes, checking once or twice and rotating the chicken to make sure it cooks evenly, but leave skin-side up.
Make a baste by stirring together the coconut milk, garlic, brown sugar, soy sauce, fish sauce and the lime zest in a bowl. Reserve the lime for squeezing the juice over the chicken once cooked.
Once the chicken has had 45 minutes skin-side up, turn it over and cook for a further 10 minutes skin-side down with the lid down. Use tongs to slide it directly over the fire and begin to brush with the baste, using a silicone pastry brush. Keep basting and turning for another 10–15 minutes until the chicken is deeply golden and cooked through. The temperature on a digital probe thermometer should read
74°C.
Lift the chicken onto a serving board then use a large knife to chop it into pieces. Squeeze over the reserved lime juice and scatter on the mint and chillies, if using.