Preheat the oven to 375°F (190°C). Lightly grease a 3-quart baking dish with butter.

Cook the elbow macaroni in salted boiling water according to package directions until al dente. Drain well and set aside.

In a large saucepan over medium heat, melt 2 tablespoons of butter. Whisk in the flour and cook for 1 to 2 minutes until the mixture turns golden and bubbly.

Gradually whisk in the whole milk and heavy cream. Cook, stirring constantly, until the sauce thickens and coats the back of a spoon, about 5 to 7 minutes.

Remove the saucepan from heat. Stir in the shredded cheddar, Gruyère, grated Parmesan, truffle oil, Dijon mustard, garlic powder, nutmeg, salt, and black pepper until completely smooth and melted.

Fold the cooked macaroni into the cheese sauce until evenly coated. Transfer the mixture to the prepared baking dish, spreading it into an even layer.

In a small bowl, combine the panko breadcrumbs with the remaining 2 tablespoons of melted butter. Sprinkle evenly over the top of the mac and cheese.

Bake for 20 to 25 minutes, or until the topping is golden brown and the sauce is bubbling around the edges.

Remove from the oven and let rest for 5 minutes. Garnish with chopped fresh parsley before serving.