Preheat the oven to 350 degrees. Bring a large pot of water to a boil. Meanwhile, in a small saucepan, melt 2½ tablespoons of the butter over low heat. Add the panko and Parmigiano-Reggiano cheese, toss well and set aside. Melt the remaining 3 tablespoons of butter in a large saucepan over low heat. Add the flour and stir frequently for 5 minutes, not letting the flour brown. Gradually whisk in the milk and cook for 5 minutes, whisking constantly. Add the salt, pepper, Gruyére and mascarpone, and continue to whisk until the cheese melts; remove from heat (mixture may be thin). Add the pasta to the boiling water and cook until al dente, about 8 minutes. Drain the pasta and return to the pot; pour the cheese sauce over the pasta and mix well. Pour the pasta mixture into a 9- by 13-inch baking dish and sprinkle with the panko mixture. Bake until golden brown and bubbly, about 30 to 45 minutes. Serves 4 to 6.