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In a large pot of boiling salted water, cook macaroni according to package instructions. Drain.
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In pot, melt 4 tablespoons butter over medium. Whisk in flour and cook, stirring frequently, until mixture is pale golden and has a nutty aroma, about 4 minutes.
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Whisking constantly, slowly add milk. Cook, whisking along bottom of pot, until boiling, about 7 minutes. Reduce heat and gently simmer, stirring occasionally, until sauce is thickened, 10 to 12 minutes. Remove from heat and add 1 tablespoon butter, mustard, Old Bay, and cheeses. Stir until cheeses are melted and sauce is smooth; season with salt. Add macaroni to sauce and stir to combine.