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Melt the butter in a large cast iron skillet over medium or high. Add the beef and black pepper and 1/2 teaspoon of the salt. Than cook stirring to crumble, until browned, about 5 minutes.
Preheat the broiler with the oven. 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; cover, reduce the heat to low, and cook, stirring it, 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. Also Garnish with the scallion, and serve immediately.