Heat 60g butter in a deep frying pan, and fry the onion until soft and golden brown.
Add the rice, stir well and cook for 15 minutes on a low heat.
Heat the stock until boiling and gradually add to the rice. Add the white wine, saffron, salt, pepper to taste and mix well. Add the cooked chicken and simmer for 20 minutes, stirring occasionally. Add more stock if the risotto become too dry.
Leave to stand for a few minutes and just before serving add a little more stock and simmer for a further 10 minutes. Serve the risotto sprinkled with the grated Parmesan cheese and the remaining butter.