Heat the oven to 375°F. Lightly grease 8 cups of a muffin pan.
In a large bowl, combine the ground chicken, egg, breadcrumbs, Parmesan, Italian seasoning, garlic powder, salt, black pepper, and ¼ cup of the marinara sauce. Mix gently until everything is evenly combined.
Divide the chicken mixture among the 8 prepared muffin cups. Press it in lightly, leaving the tops slightly rounded. Bake for 20 to 25 minutes, until the muffins are cooked through and reach 165°F in the center.
While the muffins bake, split the sub rolls. Spread the cut sides with softened butter and arrange the rolls on a baking sheet. When the muffins are cooked, place one in each roll. Spoon the remaining marinara over the muffins and sprinkle with the remaining mozzarella.
Return the assembled subs to the oven for 5 to 7 minutes, until the cheese melts and the rolls are warm. Sprinkle with chopped parsley and serve hot.