Preheat your oven to 375°F and butter a 9x13-inch baking dish.
Bring a large pot of salted water to a boil and cook the cavatappi until just shy of al dente. Drain and set aside.
In a large saucepan, melt the butter over medium heat. Whisk in the flour and cook for 1 to 2 minutes until smooth.
Slowly whisk in the warm milk, followed by the heavy cream. Continue whisking until the sauce thickens, about 5 minutes.
Whisk in the Dijon mustard, garlic powder, cayenne (if using), salt, and pepper.
Remove the pan from the heat and stir in the white cheddar, Gruyère, and Parmesan until completely melted and silky smooth.
Fold the cooked pasta into the cheese sauce until every piece is coated.
Transfer the mac and cheese to your prepared baking dish.
In a small bowl, combine the panko, melted butter, and Parmesan. Sprinkle evenly over the top.
Bake for 20 to 25 minutes, until bubbling around the edges and golden brown. For an extra crispy topping, broil for 1 to 2 minutes at the end.
Finish with fresh parsley and serve immediately.