Preheat oven to 375°F (190°C). Bring a large stockpot of heavily salted water to a boil. Cook manicotti shells for 1-2 minutes LESS than package directions. Gently drain and lay them flat on parchment paper sprayed with cooking spray.
In a large bowl, combine shredded chicken, ricotta, 1 cup mozzarella, Parmesan, parsley, egg, garlic powder, salt, and pepper. Mix until well-combined.
In a medium saucepan over medium heat, melt the butter. Add the minced garlic and sizzle for 30-60 seconds until fragrant. Sprinkle in the flour and whisk constantly for 1 minute to form a roux.
Gradually whisk in the whole milk and chicken broth until smooth. Bring to a gentle simmer and cook, stirring frequently, for 5-8 minutes until thickened. Remove from heat and stir in salt, pepper, and a pinch of nutmeg.
Spread 1 cup of the cream sauce on the bottom of a 9x13 inch baking dish. Transfer the chicken filling to a piping bag (or a Ziploc bag with a corner snipped off) and pipe the filling into each shell. Arrange shells in the dish.
Pour the remaining cream sauce over the shells and top with the remaining ½ cup of mozzarella cheese.
Bake, uncovered, for 20-25 minutes, until the sauce is bubbling and the cheese is golden brown. Let rest for 5-10 minutes before serving.