Preheat the oven to 450°F.
Bring a large pot of salted water to a boil for the pasta. Cook to al dente according to package instructions, drain, return to the pot and drizzle with olive oil to prevent sticking.
Melt the butter in a skillet over medium heat. Whisk in the flour and cook on low heat for 10 minutes.
In a separate saucepan over low heat, warm the milk. Turn off the heat and whisk the milk into the flour and butter mixture. Return the heat to medium and bring to a simmer, whisking constantly. Continue to simmer (not boil) for 15 minutes, whisking occasionally. Season with salt.
In a separate skillet, heat the oil. Add the leeks, sprinkle with salt and sauté over medium-high heat until lightly colored but not melted. Remove from the heat.
Combine the leeks, bechamel, gruyere, truffle oil and mozzarella with the cavatappi in a baking dish. Cover with breadcrumbs.
Bake for 8-10 minutes, until the cheese is melted and golden on top. Top with shaved truffles to serve.