Bring pot of salted water to a boil and cook pasta according to package directions.
In a large heavy-bottomed skillet, melt 4 tablespoons of butter over medium heat.
Add flour to melted butter and combine to form a roux. Cook for 2 minutes, whisking frequently.
Slowly add warm milk while whisking constantly to blend roux and milk.
Once milk has been added, keep whisking for 3-5 minutes until no clumps remain and sauce has thickened.
Remove from heat and add remaining Tablespoon of butter, Gruyére, and Parmesan cheeses. Stir until butter and cheeses has melted.
Combine pasta with sauce and season with salt and pepper to taste. Serve hot.
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