Preheat the oven to 350°F. Bring a large pot of water to a boil and generously season with salt.
In a large mixing bowl, combine the ricotta, mascarpone and 2 cups of the Pecorino. Stir well to combine. Stir in the salt and black pepper.
Place the pasta in the water and cook until just barely al dente, about 6 minutes. (The pasta will finish cooking in the casserole.) Drain and transfer it to the seasoned cheese mixture, stirring to coat the pasta.
Grease a 9- by 13-inch baking dish with the butter. Transfer the pasta-cheese mixture to the baking dish and top with the remaining cheese. Bake until the top and sides are crispy and golden brown, about 35 minutes. Sprinkle with more black pepper for garnish, cut into squares and serve.