Cook the Arborio rice with the chicken broth, onion, and butter in a large saucepan until the rice is tender and creamy. Season with salt and pepper, then let the risotto cool to room temperature.
Stir the grated Parmesan and truffle oil into the cooled rice until evenly combined. Beat the eggs in one bowl and place the breadcrumbs in another. Shape the rice mixture into 8 balls, dip each in egg, then roll in breadcrumbs to coat.
Heat vegetable oil in a deep skillet to 350°F. Fry the arancini in batches until golden brown on all sides, about 3 to 4 minutes per batch. Drain on paper towels.
Warm the marinara sauce in a small saucepan. Serve the hot arancini with the sauce on the side for dipping.