Preheat the oven to 375 degrees. Bring a large pot of water to a boil over high heat. Add the pasta and cook 8 minutes. Drain and transfer to a 9-by-13-inch baking dish. Drizzle the truffle oil over the warm pasta and stir to coat. 

Melt the butter in a medium saucepan over medium-high heat. Whisk in the flour and mustard and cook, whisking constantly, until there are no lumps of flour, about 2 minutes. Continue whisking and add the milk and cream gradually, then the salt and pepper. Whisk until the mixture thickens to coat a spoon, about 4 minutes. Remove from the heat and stir in the cheddar, gruyere, and ¾ cup of parmesan. Pour the sauce over the pasta in the baking dish and stir carefully to coat the pasta. Top with the breadcrumbs and then ¼ cup of the parmesan. Bake uncovered for 30 minutes, then let the macaroni rest for 10 minutes. Sprinkle with chives before serving.