Bring a large pot of salted water to a boil. Cook elbow macaroni al dente according to package directions. Drain and set pasta aside in the large pot.
In a medium saucepan melt the butter over medium heat. Add flour and stir to create a roux. Cook for 1-2 minutes.
Gradually whisk in the milk, one cup at a time. Keep cooking while stirring until thickened.
Remove from heat and stir in 3 cups of shredded cheese until melted and fully combined. Stir in the sour cream. Taste, and season with salt & pepper as desired.
Pour sauce into the cooked macaroni and stir to combine.
Pour mixture into a greased 3-quart baking dish and top with remaining shredded cheese.
Bake for 15-20 minutes or until cheese is melted and bubbly. Broil for 2-3 minutes to brown the cheese if desired.
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Notes
Leftover mac and cheese will keep in an airtight container in the refrigerator for 3-5 days.