Bring a large pot of salted water to a boil. Add pasta and cook until al dente, about 8-10 minutes. Drain and set aside.

In a medium saucepan, melt 4 tablespoons of unsalted butter over medium heat. Whisk in flour and cook, stirring constantly for about 1 minute.

Gradually add heavy cream, whisking to prevent lumps. Cook for 3-4 minutes until thickened. Lower heat and fold in Boursin cheese and white cheddar until melted.

Stir cooked pasta into the cheese sauce until fully coated. Allow to rest off heat for a few minutes.

In a skillet, melt remaining 2 tablespoons of butter. Add breadcrumbs and sauté for 3-4 minutes until golden brown.

Transfer pasta mixture to a baking dish, sprinkle breadcrumbs on top. Serve immediately or broil for a few minutes for a crispy topping.