Position a rack about 6 to 8-inches, from the broiler and preheat.
Bring a large pot of water to a boil, salt it generously, and add the elbow pasta and cook until tender but still slightly more firm than al dente (about 5 minutes). Drain the pasta in a colander set in the sink; don't rinse.
Meanwhile, melt 1/4 cup butter in a large saucepan over medium heat. Scatter the flour over the butter and mix with a wooden spoon into a paste. Continue stirring in a figure "8" making sure to get into the corners of the pan, until the paste puffs slightly and lightens in color, about 1 minute. Off the heat gradually whisk the milk into the paste. Return to medium-high heat and whisk until thickened, then simmer, stirring occasionally until the sauce has the consistency of heavy cream, about 8 to 10 minutes. Season with salt, pepper and nutmeg. Whisk in mustard. On the lowest heat setting, whisk 3 cups of the cheese into the sauce alternating with chicken stock.
Toss the hot pasta with the cheese sauce in a large bowl to coat evenly. Transfer to a dutch oven.
Mix the panko breads crumbs, melted butter, and remaining 1 cup cheddar together; scatter over the macaroni.
Place macaroni and cheese under the broiler and cook until hot and bubbly and the breadcrumbs are brown and toasty, about 2 to 4 minutes. Let cool 10 minutes and serve.
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