Melt half the butter in a saucepan over a medium heat. Add the pancetta and onion and cook for 10 minutes. Cut the chestnuts in half and add to the pan with the rice and stock. Bring to the boil, then reduce the heat and simmer for 20 minutes, or until most of the liquid has been absorbed and the rice is tender.
Heat the milk in a small saucepan until tepid, then stir into the rice with the remaining butter and season the dish with salt and pepper. Cover and leave to stand for 5 minutes before serving.