Melt he butter in a large cast-iron skillet over medium-high heat. Once hot, add the beef and crumble with a wooden spoon, cooking until browned, about 5 minutes. Stir in the onion, garlic, jalapeño (if using), 1/2 teaspoon salt and pepper, and cook for another 3 minutes.

Reduce stovetop heat to medium, and stir in the macaroni. Add the milk, water, mustard, paprika, garlic powder, cayenne, 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 melted through and the sauce has thickened. Stir in the chives, and give it a taste. Adjust seasoning as needed, adding more salt or pepper. Sprinkle the panko over the top and place under a preheated broiler until the top is golden brown, 30 seconds to 1 minute. Garnish with scallions and serve immediately. Enjoy!