Step 1: Preheat your oven to 375°F (190°C). Grease a 9×13 inch baking dish.
Step 2: Cook the pasta according to package directions until al dente. Drain well and set aside.
Step 3: In a large, heavy-bottomed saucepan or Dutch oven, melt the 6 tablespoons of unsalted butter over medium heat. Once melted, whisk in the flour and cook for 1-2 minutes, stirring constantly, until a smooth roux forms and smells slightly nutty (do not let it brown).
Step 4: Gradually whisk in the warm milk, about 1/2 cup at a time, ensuring each addition is fully incorporated before adding more. Continue whisking until the sauce is smooth and lump-free. Bring the sauce to a gentle simmer, stirring constantly, until it thickens enough to coat the back of a spoon, about 5-7 minutes.
Step 5: Remove the saucepan from the heat. Stir in the salt, white pepper, nutmeg, and Dijon mustard.
Step 6: Add the grated cheddar, Gruyère, and Parmigiano-Reggiano cheeses in batches, stirring until each batch is completely melted and the sauce is smooth. Ensure the heat is off to prevent the cheese from seizing.
Step 7: Stir in 1.5 to 2 tablespoons of the truffle oil. Taste and adjust seasoning if necessary.
Step 8: Add the drained, cooked pasta to the cheese sauce and stir gently until all the pasta is evenly coated.
Step 9: Pour the Truffle Mac and Cheese mixture into the prepared baking dish. If desired, combine panko breadcrumbs with 2 tablespoons melted butter and sprinkle evenly over the top.
Step 10: Bake for 20-25 minutes, or until bubbly and golden brown on top. Let stand for 5 minutes before serving.
Step 11: Drizzle with the remaining 1/2 to 1 tablespoon of truffle oil just before serving. Garnish with fresh parsley or chives, if using. Enjoy your delicious Truffle Mac and Cheese!