01 - Preheat oven to 400°F. Lightly grease a medium baking dish (approximately 2 quarts) with butter.
02 - Boil elbow macaroni in a large pot according to package directions until al dente. Drain well and set aside.
03 - Melt 2 tablespoons butter in a medium saucepan over medium heat. Whisk in flour and cook for about 1 minute, stirring constantly.
04 - Gradually whisk in 2 cups milk, stirring continuously until smooth and slightly thickened, 3–4 minutes.
05 - Remove sauce from heat. Stir in Gruyère, cheddar, Parmesan, Dijon mustard, salt, black pepper, and 1 1/2 tablespoons truffle oil until cheeses are fully melted and sauce is creamy.
06 - Add drained macaroni to the cheese sauce and mix until evenly coated.
07 - Spoon the combined macaroni and cheese sauce into the prepared baking dish.
08 - Mix panko breadcrumbs with melted butter and 2 tablespoons Parmesan cheese in a bowl. Evenly distribute over macaroni.
09 - Place dish in the preheated oven and bake for 15–18 minutes until the topping is golden brown and the filling bubbles.
10 - Let rest for 5 minutes. Drizzle with extra truffle oil and optional fresh parsley before serving.