1. To make the marinade, place the rice in a wok over a medium-high heat and toast for 5 minutes. Then add the coconut and continue toasting for 10 more minutes until both the rice and coconut are deep golden brown. Be careful not to burn them.
2. Once cooled, transfer to a pestle and mortar or spice grinder and pound to a fine powder. Add the remaining marinade ingredients and pound or grind to a paste. Rub the paste all over the chicken, then cover and refrigerate overnight.
3. When ready to cook, preheat the oven to 200°C/180°C fan/400°F. Place the chicken on a large roasting tray lined with foil and roast for 40 minutes, then increase the heat to 220°C/200°C fan/430°F and cook the chicken for a further 15 minutes. Check that the chicken is cooked by piercing the thickest part of the thigh with a knife. The juices should run clear; if not, return the chicken to the oven covered with foil for a further 5 minutes and check again. Once cooked, remove from the oven and rest for 10 minutes, before carving and serving with any roasting juices from the tray.
4. For the ideal Sri Lankan roast, serve alongside a sambol or mallung, some curry sauce and pol rotis.