Bring a large pot of salted water to a boil. Add the pasta and cook as the label directs. Reserve 1 cup of the cooking water; drain the pasta.
Meanwhile, cook the bacon in a large skillet over medium heat, stirring occasionally, until crisp, about 8 minutes. Remove with a slotted spoon and drain on paper towels; pour off all but 1 tablespoon of the drippings from the skillet.
Add the leeks to the drippings in the skillet. Season with 1/2 teaspoon each salt and pepper and cook, stirring occasionally, until soft, about 3 minutes. Add the heavy cream and cook until it begins to thicken, about 2 minutes.
Add the pasta to the skillet along with the parmesan, half of the bacon and 1/2 teaspoon pepper; toss to coat, adding enough of the reserved pasta cooking water to loosen. To serve, top with the remaining bacon, more parmesan and parsley.