To form the arancini, prepare a small bowl of water. Cut the mozzarella cheese into 1 cm cubes. You will need about 25 cubes of cheese. Using wet hands, scoop up a golf ball sized amount of cold risotto. Press out flat on the palm of one hand. Place a cube of mozzarella in the middle of the risotto. Bring up the edges and cover the cheese. Gently roll the ball around to smooth the edges. If the risotto is sticking, wet your hands again. Place the filled rice balls on a baking tray and set aside.
Prepare a standard breading stating using three shallow dishes. In one dish add the all-purpose flour with a generous pinch of kosher salt. In another dish add the eggs and water and whisk together until blended. In the final dish add the panko crumbs with a generous pinch of kosher salt.
Bread the arancini by first gently rolling in the flour. Transfer to the egg wash and gently roll to coat the ball. Transfer to the panko crumbs and cover with the crumbs. Gently press the crumbs on the rice ball being careful not to crush it. Transfer the coated arancini to a baking sheet and set aside. At this point the arancini can be covered and stored in the fridge for up to a day.
To fry the aranacini, add enough vegetable oil to a deep skillet or pot to completely cover the arancini, about 2 inches. Heat over medium high heat until the oil reaches 325°F / 162°C. Carefully lower the arancini into the hot oil with a spoon. Fry about 5 to 6 arancini at one time. Gently stir the arancini with a slotted spoon while frying so they cook evenly.
Fry the arancini for 5 to 7 minutes until well browned. Remove from the oil with a slotted spoon onto a paper towel lined baking sheet. Repeat with the remaining arancini until they are all cooked. If desired, keep the arancini warm in a low oven while frying.
To serve, transfer the arancini to a platter. Sprinkle with chopped parsley and freshly grated parmesan cheese. Serve with the warmed marinara sauce and enjoy!