Melt the butter in a large cast-iron skillet over medium-high. Add the beef, black pepper, and 1/2 teaspoon of the salt; cook, stirring to crumble, until browned, about 5 minutes.
Preheat the broiler with the oven rack 3 to 4 inches from the heat. Reduce the stovetop heat to medium, and stir in the macaroni. Add the milk, warm water, mustard, paprika, garlic powder, cayenne pepper, and remaining 1/2 teaspoon salt.
Bring to a low simmer; cover, reduce the heat to low, and cook, stirring occasionally, until the macaroni is tender, about 12 minutes. Add the cheeses, and cook, stirring constantly, until the cheeses melt. Stir in the chives. Sprinkle the panko over the macaroni mixture. Broil until the panko is golden brown, 1 to 2 minutes. Garnish with the scallion, and serve immediately.
Variation: Add some slices of jalapeño for extra kick.
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