Form leftover risotto into balls ~2 inches in diameter. When forming balls, place a small piece of cheese into the center of the ball.
Refrigerate risotto balls for at least 30 minutes prior to cooking.
When you are ready to cook the arancini, place breadcrumbs on a large plate and season with salt and pepper.
Dip arancini into egg, and then roll in breadcrumbs to coat.
Heat oil in a cast iron skillet or frying pan on medium heat until a breadcrumb dropped into the oil immediately floats to the surface.
Fry arancini, turning to cook all sides until heated through and golden brown, ~3 minutes.
Drain on paper towels and serve immediately.