Boil a large pot of salted water. Add pasta and cook until al dente, about 7-9 minutes. Drain and reserve a cup of pasta water.

Melt 4 tablespoons of butter in the same pot over medium heat. Whisk in 4 tablespoons of flour to create a roux, cooking for 1-2 minutes until golden.

Gradually pour in 2 cups of heavy cream while whisking continuously. Add 1 cup of Boursin and 1 cup of cheddar, stirring until melted and creamy.

Add the cooked pasta to the cheese sauce, stirring until coated. Add reserved pasta water to achieve a creamy consistency if needed.

Melt 1 tablespoon of butter in a skillet. Add breadcrumbs and toast for 3-5 minutes until golden brown and fragrant.

Preheat oven to 350°F. Pour the pasta and cheese mixture into a greased baking dish, top with breadcrumbs, and bake for 20-25 minutes until bubbly and golden.