Bring stock to a simmer. Toast arborio rice in olive oil for 2 minutes, then add stock a ladle at a time, stirring often, for 18 minutes until thick and creamy. Stir in Parmesan, parsley, salt, and pepper. Cool 10 minutes, beat in egg yolks, then chill the rice mixture until cold and firm, at least 1 hour.

Shape 2-tablespoon portions of cold rice around a mozzarella cube, rolling tightly to seal. Wet hands to prevent sticking. Chill the shaped balls on a tray.

Set up flour, beaten egg, and a mix of plain and panko breadcrumbs. Coat each ball in flour, egg, then breadcrumbs, pressing gently. Rest for 10 minutes.

Heat 2 inches of oil to 350°F (175°C). Fry in batches for 4 to 5 minutes, turning occasionally, until deep golden brown.

Drain on a rack and serve immediately with warm marinara sauce.