Preheat oven to 350 degrees. Grease a 9 x 13 baking dish and set aside.
Melt butter in a large sauce pan over medium-low heat.
Add the flour and whisk until smooth.
Slowly add milk, whisking continuously.
Add the salt and white pepper and bring to a boil.
Stir constantly, until the mixture thickens enough that it coats the back of a spoon.
Mix in 1 cup white cheddar, gruyere, cream cheese, and sour cream. Stir until melted.
Immediately remove from heat.
Pour the cooked elbow macaroni into the cheese sauce and mix to coat evenly.
Pour the cheese-coated noodles into the prepared baking dish.
Top with remaining 1/2 cup white cheddar cheese.
Bake for 15 minutes or until the cheese has melted and the edges bubble.
Serve and enjoy!