Preheat an oven to 375°F. Butter a 9-by-13-inch baking dish.
Bring a large saucepan of salted water to a boil over high heat. Add the macaroni and cook, stirring occasionally, until not quite al dente, about 2 minutes less than the package instructions. Drain and transfer to a large bowl. While the pasta is still warm, drizzle with the truffle oil and stir well.
Return the saucepan to medium-high heat and melt the 4 Tbs. butter. Add the flour, paprika and mustard and cook, stirring well, until no visible flour remains, 1 to 3 minutes. Whisk in the milk, half-and-half and a generous pinch of salt and bring to a boil. Simmer, whisking frequently to smooth out any lumps, for 4 to 5 minutes. Remove from the heat. Add a pinch of pepper and 1 cup each of the Gruyere and cheddar. Stir until smooth.
Pour the cheese sauce onto the prepared macaroni, add the minced chives or green onions and stir well. Transfer to the prepared baking dish and top with the remaining cheeses. Bake until the top is lightly browned and the sauce is bubbly, 25 to 30 minutes. Let stand for 5 minutes before serving.