Cook the Pasta: Bring a large pot of salted water to a boil. Add pasta and cook until al dente, about 7-9 minutes. Drain and set aside.

Prepare the Roux: Melt butter in the same pot, whisk in flour to create a roux, cooking for 2-3 minutes until slightly bubbly.

Make the Cheese Sauce: Gradually pour in heavy cream while whisking, allowing it to thicken for about 3-4 minutes.

Incorporate the Cheeses: Stir in Boursin cheese and white cheddar, whisking until melted and smooth. Add reserved pasta water or milk if too thick.

Combine Pasta and Sauce: Add drained pasta to cheese sauce, folding gently until coated with the sauce.

Prepare the Breadcrumb Topping: Melt remaining butter in a skillet, add breadcrumbs, and toast for 3-5 minutes until golden brown.

Serve and Enjoy: Transfer to a serving dish, sprinkle with toasted breadcrumbs, and serve hot, optionally garnished with parsley.