Creating Chicken Cordon Bleu Casserole is simple when you follow these steps:

1. Preheat Oven: Start by preheating your oven to 350°F (175°C).
2. Cook Pasta: Cook rotini pasta according to package instructions until al dente, then drain and set aside.
3. Mix Ingredients: In a large bowl, combine cooked chicken, ham, rotini pasta, and shredded Swiss cheese.
4. Prepare Sauce: In a separate bowl, mix together sour cream, cream of chicken soup, milk, Dijon mustard, garlic powder, salt, and pepper until well blended.
5. Combine: Pour the sauce over the chicken mixture and stir until everything is evenly coated.
6. Transfer to Baking Dish: Grease a 9×13 inch baking dish and pour the combined mixture into it.
7. Prepare Topping: In a small bowl, mix breadcrumbs with melted butter. Sprinkle over the casserole to create a crunchy topping.
8. Bake: Place the baking dish in the preheated oven and bake for 35-40 minutes, or until the casserole is hot and bubbly.
9. Cool Slightly: Once done, allow it to cool for about 5-10 minutes before serving. This will help the casserole set a bit.
10. Serve: Spoon portions onto plates or bowls, garnishing with fresh parsley if desired.

These steps will guide you in putting together this incredible casserole that everyone will rave about!