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1
For the topping, process the bread and butter in a food processor fitted with the steel blade until coarsely ground, about six 1-second pulses; set aside.
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2
For the mac and cheese, adjust an oven rack to the middle position and heat the oven to 400.
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3
Bring 4 quarts of water to a boil in a Dutch oven over high heat.
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4
Stir in 1 tablespoon salt and macaroni; cook, stirring occasionally, until al dente, about 5 minutes.
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5
Drain the pasta and leave it in the colander; set aside.
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6
Wipe the pot dry; add in the butter and return to medium heat until melted.
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7
Add in the garlic, mustard, and cayenne; cook until fragrant, about 30 seconds.
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8
Add in the flour and cook, stirring constantly, until golden, about 1 minute.
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9
Slowly whisk in the chicken broth and milk; bring to a simmer and cook, whisking often, until large bubbles form on the surface and the mixture is slightly thickened, 5-8 minutes.
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10
Off the heat, whisk in the colby and cheddar cheese gradually until completely melted.
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11
Season with salt and pepper to taste.
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12
Add the drained pasta to the cheese sauce and stir, breaking up any clumps, until well combined.
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13
Pour into a 13 x 9 inch baking dish and sprinkle with the bread-crumb topping.
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14
Bake until golden brown and bubbling around the edges, 25-30 minutes.
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15
Remove from oven and cool for 10 minutes before serving.
