Take a spoonful of risotto and fill it with a cube of mozzarella. Shape the risotto into a small ball. Continue the process until all the risotto is used up.
In a small bowl, mix the flour with a generous pinch of sea salt and freshly-cracked black pepper. Place the eggs in a separate bowl and whisk until light and creamy, then place the panko breadcrumbs in another bowl.
Dip each risotto ball into the flour, then into the egg mixture and then into the breadcrumbs, evenly covering it all around. Place the prepared ball on a tray and repeat until all arancini are ready. Store in the refrigerator unti ready to fry them.
Heat the vegetable oil into a medium saucepan until 170°C/350°F, then fry the arancini in small batches for about 2 min, until crispy and golden-brown on all sides.
Remove the rice balls from the pan with a slotted spoon and transfer on a plate covered with kitchen paper to aborb the oil in excess, then serve. pat them dry on kitchen paper, and serve immediately.