Position a rack in the middle of the oven and preheat to 425°F (220°C). Generously butter a half-sheet pan and line with parchment paper.

In a large bowl, mix together the cheddar and Gruyère. Measure out 3 cups (375 g) of the cheese mixture and set aside for the topping. Set the large bowl with the remaining cheese near the stovetop.

In a small bowl, combine the panko, Parmigiano-Reggiano, melted butter, salt, and cayenne. Generously season with black pepper and stir until the bread crumbs are evenly coated.

In a large pot, combine the pasta, milk, and 21/2 cups (600 ml) water. Set the pot over medium-high heat and bring to a lively simmer, stir- ring occasionally. Reduce to a gentle simmer and cook, stirring often, until al dente, 3 to 4 minutes. Remove from the heat and stir in the smoked paprika. Add the cheese mixture from the large bowl, one handful at a time, stirring to melt after each addition. Generously sea- son with black pepper and stir to combine.

Transfer the mixture to the prepared sheet pan and spread in an even layer. Cover with the reserved cheeses and sprinkle with the bread crumb mixture. Bake for 20 to 30 minutes, rotating the pan front to back halfway through, or until deeply golden and crispy. Let sit for 10 minutes before serving.