Line a rimmed baking sheet with parchment paper. Set aside.

Scoop about 3 tablespoons of chilled risotto into your hand and flatten it into a disk. Place one cube of mozzarella in the center, press the risotto firmly around it, and roll into a tight ball. Transfer to the prepared baking sheet. Repeat with the remaining risotto and cheese to make 10 balls.

Set up a breading station with three shallow bowls. Whisk the flour, salt, and pepper in the first, add the beaten eggs to the second, and add the breadcrumbs to the third.

Working one at a time, dredge each ball in the seasoned flour and shake off the excess. Dip in the egg, letting the excess drip off. Roll in the breadcrumbs, pressing gently to coat evenly. Return to the baking sheet.

Pour about 1 inch of oil into a 10-inch cast iron skillet and heat over medium-high heat to 350°F.

Working in batches of 3 to 4, carefully lower the arancini into the oil. Fry, turning with tongs, until deep golden brown on all sides, about 6 to 8 minutes total. Transfer to a paper towel-lined plate.

Repeat with the remaining arancini, letting the oil return to 350°F between batches.

Serve warm with marinara sauce.