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Add the butter to a pan and cook until melted.
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Whisk in the flour and cook about 3 minutes until it slightly darkens in color.
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Whisk in the milk and bring to a simmer, whisking often. You will notice it thicken slightly when it simmers.
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Remove from heat and stir in the cheese.
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Meanwhile, cook the pasta al dente and strain, reserving a cup of pasta water. Add to a bowl.
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Pour the cheese sauce over the pasta and stir to combine. Add a little pasta water if needed to loosen it up.
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Add some of the mac and cheese to an oven safe baking dish. Top with carnitas and beans. Add more pasta. Bake at 350 for 20 minutes. Remove from oven and top with salsa and sour cream. Serve.