Preheat the oven to 350 degrees. Melt 2 tablespoons of the butter in a large skillet. Add the leeks and cook over high heat, stirring just until slightly wilted, 5 minutes; season with salt and pepper. Cook over low heat until very tender, about 20 minutes.
Meanwhile, in a large saucepan, melt the remaining 2 tablespoons of butter. Add the flour and cook over moderately high heat, whisking, for 2 minutes. Add the half-and-half and bring to a boil, whisking until thickened, about 5 minutes. Remove from the heat. Add 2 cups of the cheese, season with salt and pepper and whisk the cheese sauce until melted.
In a large saucepan of salted boiling water, cook the macaroni until nearly al dente. Drain well. Add the macaroni and the cheese sauce to the leek greens and stir until combined.Transfer the macaroni to an 8-by-11-inch baking dish and sprinkle with the remaining 1/2 cup of cheese. Bake for about 40 minutes, until bubbling. Turn the broiler on and broil the mac and cheese until golden brown on top, about 3 minutes. Let stand for 15 minutes before serving.
