Rinse the chicken in cold tap water and put it into a bowl. Add the milk, salt, black pepper and rosemary leaves then mix them well. Cover the bowl and leave it in the fridge for about 20 minutes. Drain off the milk. (Leave any rosemary residue on the chicken as it is. It will add a nice aroma once baked.)
Put the chicken into a large zipper lock bag (27cm x 33 cm/10.6 inch x 13 inch) and add the sauce. Seal the bag and vigorously shake the bag and massage the chicken so that the sauce smears well into the chicken. Alternatively, you can use a brush to paste the sauce onto the chicken. Marinate it for at least 4 hrs in the fridge.
Preheat the oven at 240 C/ 464 F (on the fan forced setting) for 20 mins. Take the chicken out from the fridge. Lay down a baking paper on top of the oven tray and line up the chicken. Make sure they are not stacked up on top of each other.
Put the tray into the oven and bake it at 240 C/ 464 F for 10 mins. Take the tray out and turn the chicken over and put it back in the oven. Bake it a further 10 mins or until charred on the skin.
Serve. (Optionally, garnish with some sesame seeds and/or green onion.)