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In saucepan, melt butter. Remove from heat.
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Blend in flour, salt and mustard, until smooth.
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Over the heat, add milk slowly, and mix, stirring constantly, until the sauce thickens a little and is smooth.
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Add 1 ½ cups of the cheese and heat until melted, stirring occasionally.
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Meanwhile, cook macaroni according to the package. Drain.
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Combine the macaroni with the sauce into a 2-quart casserole.
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Top with the remaining cheese, breadcrumbs and a bit of paprika.
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Bake at 375°F for about 20 to 25 minutes or until nicely browned and bubbly.