Bring a large pot of water to a rolling boil. Add 1 tablespoon kosher salt. Add 12 oz penne and cook until al dente according to package directions (usually 9–11 minutes). Reserve 1 cup of pasta cooking water, then drain pasta and set aside.

Cheese sauce

Make a roux

In a medium heavy‑bottomed saucepan over medium heat, melt 3 tablespoons unsalted butter until foaming. Add 3 tablespoons all‑purpose flour and whisk constantly for 1–2 minutes until the raw flour smell is gone and the mixture is lightly golden.

Add milk and cream

Slowly pour in 2 cups whole milk and 1 cup heavy cream while whisking to prevent lumps. Increase heat to medium‑high and bring to a gentle simmer, whisking occasionally, until the mixture thickens enough to coat the back of a spoon (about 4–6 minutes).

Season and finish sauce

Lower heat to medium‑low. Stir in 1 teaspoon Dijon mustard, 1/2 teaspoon Worcestershire sauce, 1/8 teaspoon ground nutmeg, and a pinch of salt. Add the grated cheeses in two batches: first 1 cup Gruyère, whisk until melted and smooth, then 1 cup sharp cheddar and 1/2 cup grated Parmesan; stir until fully incorporated and the sauce is silky. If the sauce is too thick, whisk in up to 1/2 cup reserved pasta water (add a tablespoon at a time) to reach desired consistency. Taste and adjust salt and pepper.

Combine & bake (if using topping)

Preheat the oven to 375°F (190°C) if using the breadcrumb topping. In a large bowl or the skillet with the sauce off the heat, add the drained penne to the cheese sauce and toss to coat evenly. If needed, add a splash of reserved pasta water so the sauce clings to the pasta (about 2–4 tablespoons).

If serving stovetop, transfer to warmed serving bowls and proceed to finishing. To bake: lightly butter a 9x9 inch baking dish or similar, pour the mac & cheese into the dish and spread evenly.

Prepare topping: in a small bowl combine 1/2 cup panko breadcrumbs, 1 tablespoon melted butter, and 2 tablespoons extra Parmesan. Sprinkle evenly over the pasta. Bake for 12–15 minutes until bubbly and golden on top. For a deeper color, broil 1–2 minutes watching carefully.

Truffle & finishing

When ready to serve, shave the fresh black truffle thinly over each portion (reserve a few shavings for garnish). If using truffle oil, drizzle 1 teaspoon over the mac & cheese for extra aroma. Sprinkle with freshly ground black pepper and 1 tablespoon chopped parsley. Serve immediately.