Start by placing an oven rack in the top half of your oven. Preheat the broiler to high, preparing for that final bubbly finish.
Next, bring a large pot of salted water to a rolling boil. Once boiling, add the elbow pasta and cook for about 5 minutes, just until it’s barely al dente. You want it to hold its shape since it will continue cooking in the oven. After cooking, strain the pasta and set it aside.
In a 10-inch cast iron skillet or an oven-safe saucepan, heat the unsalted butter over medium-high heat. Allow the butter to melt completely until it becomes foamy, signaling readiness for the next step.
Sprinkle the all-purpose flour over the melted butter, whisking continuously to incorporate it into a smooth paste. Keep whisking until the mixture becomes frothy and turns a light golden brown, which should take about 2 to 3 minutes.
Gradually drizzle in the whole milk, whisking vigorously to prevent any lumps from forming. If clumps develop, keep whisking until the mixture is smooth before adding more milk.
Continue whisking until all the milk is added to the mixture. Cook this mixture, whisking regularly, for about 7 to 8 minutes until it thickens to a consistency resembling heavy cream, and small bubbles just begin to simmer around the edges.
Now, whisk in the cayenne pepper, paprika, and a pinch of salt and black pepper to taste. Once combined, remove the skillet from heat.
Set aside about 1/4 cup of your cheese mixture (a blend of cheddar, Gruyere, and Pecorino) for topping later. Slowly add the remaining cheese to the skillet in handfuls, whisking well until each handful is melted and smooth.
If your sauce has any noticeable chunks, return the pan to low heat and whisk for no more than 1 to 2 minutes until the cheese is completely melted and the sauce is velvety.
Finally, fold the cooked macaroni into the cheese sauce until it's all evenly coated. Sprinkle the reserved shredded cheese and Panko bread crumbs over the top.
Broil your skillet on high for about 4 to 5 minutes, watching closely until the cheese is bubbly and the breadcrumbs turn golden brown. Once done, remove from the oven and let cool for about 5 minutes before serving.