Preheat the oven to 375°F (190°C) and lightly grease a muffin tin or line it with paper liners.
In a large mixing bowl, combine the ground chicken, breadcrumbs, grated Parmesan cheese, chopped parsley, minced garlic, chopped onion, egg, Italian seasoning, salt, and black pepper.
Add the milk to the mixture to keep the meatloaves moist, and mix gently with your hands or a wooden spoon until everything is well combined but not overmixed.
Scoop the chicken mixture into the prepared muffin tin, filling each cup to about 3/4 full to allow for expansion.
Spoon a generous tablespoon of marinara sauce on top of each mini meatloaf.
Bake in the preheated oven for 25-30 minutes or until the internal temperature reaches 165°F (75°C) and the tops are golden brown.
Remove from the oven and let cool for 5 minutes before carefully removing the mini meatloaves from the muffin tin.
Garnish each with fresh basil leaves before serving.