In a medium bowl, combine ground beef, breadcrumbs, Parmesan cheese, beaten egg, minced garlic, milk, salt, and black pepper. Mix until well combined.
Form the mixture into meatballs, about 1-1.5 inches in diameter.
In a large skillet, heat olive oil over medium heat. Add the meatballs and cook until browned on all sides, about 8-10 minutes.
Once browned, add the marinara sauce to the skillet. Reduce heat to low and simmer for 20 minutes, or until meatballs are cooked through.
While the meatballs are cooking, bring a large pot of salted water to a boil. Cook the spaghetti according to package instructions. Drain well.
Serve the meatballs and sauce over the cooked spaghetti. Garnish with fresh basil and additional Parmesan cheese, if desired.