Put the stock and saffron in a large saucepan and bring to the boil, then reduce the heat to low and leave to simmer gently.
Meanwhile, melt the butter in a medium pan over a low heat, add the rice and stir well until all the grains are coated in butter. Add a ladleful of the hot stock and simmer gently, stirring constantly, until it has all been absorbed. Continue adding stock in this way for 20-25 minutes, until the rice is tender and creamy but still al dente.
Remove from the heat and stir in the egg yolk, grated cheese and some seasoning to taste. Pour into a shallow baking tray, press cling film onto the surface and leave to go cold, then chill for 2 hours.
Make the arancini. Using slightly wet hands, divide the risotto into palm sized pieces. Take 1 piece of risotto and flatten to a small disc. Put a large pinch of chopped ham and a few pieces of mozzarella into the centre and shape the rice around the filling into a ball, patching up any gaps with a little more risotto, if necessary. Repeat to make about 12
Heat a deep-fat fryer to 180°C. Coat the risotto balls in beaten egg, then breadcrumbs and deep-fry in batches for 3-4 minutes, until crisp and golden. Drain on kitchen paper and serve hot.