1. In a saucepan, bring the chicken broth to a simmer. Add the arborio rice, butter, salt, and pepper. Cook on low heat for 15-18 minutes, stirring occasionally, until the rice is tender and the liquid has been absorbed. Stir in the grated Parmesan cheese. Spread the cooked rice on a baking sheet and refrigerate for 1-2 hours until completely chilled.

2. Preheat your oven to 400°F. Line a baking sheet with parchment paper to prepare for the baked arancini.

3. Take a handful of the cold rice and flatten it in the palm of your hand. Place a cube of mozzarella cheese (and any optional filling) in the center. Gently shape the rice around the cheese, ensuring it is completely enclosed. Repeat this process with the remaining rice to form the arancini balls.

4. Set up three separate bowls: one with the all-purpose flour, one with the beaten eggs, and one with the panko breadcrumbs. Gently roll each arancini ball in the flour, then dip it in the beaten egg, and finally coat it with the panko breadcrumbs. Place the breaded arancini on the prepared baking sheet.

5. Lightly drizzle or spray the arancini with olive oil. Bake in the preheated oven for 20-25 minutes, flipping them halfway, until they are golden brown and crispy.