1. Preheat your oven to 375 F (190 C). In a large 9×13-inch baking dish, combine the shredded cooked chicken, marinara sauce, chicken broth, Italian seasoning, garlic powder, and black pepper. Stir everything together until well combined. The mixture should be saucy and evenly distributed in the dish.

2. In a medium bowl, whisk together the all-purpose flour, baking powder, and salt. In a separate small bowl, combine the milk and melted butter. Pour the wet ingredients into the dry ingredients and stir with a spoon until just combined. Be careful not to overmix; a few lumps are perfectly fine and will lead to tender dumplings.

3. Using a tablespoon or a small cookie scoop, drop spoonfuls of the dumpling batter evenly over the chicken and sauce mixture in the baking dish. Aim for about 12-15 dumplings, leaving a little space between each one as they will expand during cooking.

4. Sprinkle the shredded mozzarella cheese generously over the dumplings and the exposed sauce. Then, sprinkle the grated Parmesan cheese over the mozzarella. This creates a delicious, bubbly cheese crust.

5. Place the baking dish in the preheated oven and bake for 20-25 minutes. The casserole is done when the sauce is bubbly, the cheese is melted and lightly golden, and the dumplings are cooked through and fluffy. You can insert a toothpick into a dumpling; if it comes out clean, they are ready.

6. Once baked, remove the casserole from the oven and let it rest for 5 minutes before serving. This allows the sauce to set slightly and prevents it from being too runny. If desired, garnish with fresh basil leaves before serving for a pop of color and fresh flavor.