In a large mixing bowl, combine ground beef, Italian bread crumbs, grated Parmesan cheese, egg, dried oregano, dried basil, onion powder, garlic powder, kosher salt, and black pepper. Mix until just combined, then form into 1-inch meatballs.

Heat a large skillet over medium heat. Add meatballs and cook for 8–10 minutes, turning occasionally until browned. Drain excess grease and pour in marinara sauce. Simmer for 15 minutes.

Preheat broiler. Spread butter on cut sides of sub rolls and place on a baking sheet. Broil for 2–3 minutes until golden.

Remove rolls from the oven. Place a handful of mozzarella cheese on the bottom of each roll. Add five meatballs to each roll and top with remaining mozzarella.

Return assembled subs to broiler for 2–3 minutes until cheese is bubbly.

Let subs sit for a minute, then serve hot.