Bring a large pot of lightly salted water to a boil. Add penne and cook, stirring occasionally, until tender yet firm to the bite, about 11 minutes. Drain.
Bring a second large pot of lightly salted water to a boil and cook leek for 2 minutes while penne is cooking. Drain well.
Preheat oven to 475 degrees F (245 degrees C). Grease a baking dish with some butter.
Heat cream and butter in a saucepan over low heat until hot. Whisk in Emmentaler cheese until melted. Season with salt and pepper. Mix cooked penne and leek and add to the prepared baking dish. Pour cheese sauce on top.
Bake in the preheated oven until lightly browned, about 10 minutes.
