Mix meatball ingredients:
In a large bowl, combine ground chicken, egg, Parmesan, breadcrumbs, garlic powder, Italian seasoning, salt, black pepper, and optional red pepper flakes. Mix gently with hands or a spoon until just combined; do not overwork.Stuff with mozzarella:
Scoop mixture and roll into golf-ball-sized portions. Press a small cube of mozzarella into the center of each ball, then seal completely so no cheese leaks out.Coat in breadcrumbs:
Roll each stuffed meatball in extra breadcrumbs to coat fully on all sides for a crispy crust.Brown in skillet:
Heat olive oil in a skillet over medium heat. Cook meatballs in batches, turning gently, until golden brown and crisp on all sides.Bake with sauce and cheese:
Preheat oven to 350°F (175°C). Pour marinara sauce into the skillet or a baking dish and arrange meatballs over sauce. Bake for 20 minutes. During the final 5 minutes, sprinkle shredded mozzarella over meatballs and return to oven until melted.Garnish and serve:
Remove from oven. Garnish with fresh basil and extra Parmesan. Serve over pasta or with garlic bread.