1. Preheat oven to 375°. Cook bacon in a skillet over medium heat, stirring occasionally, 6 to 8 minutes or until crisp; remove bacon, and drain on paper towels. Discard drippings.
2. Cook leeks in 4-qt. boiling water in a large Dutch oven 5 minutes. Remove leeks with a slotted spoon, reserving boiling water in Dutch oven. Plunge leeks into ice water to stop the cooking process; drain.
3. Add pasta to boiling water, and cook 10 minutes or until al dente. Drain.
4. Melt butter in a large skillet over medium heat. Reduce heat to medium-low, and whisk in flour until smooth; cook, whisking constantly, 2 minutes or until golden brown. Slowly whisk in milk, and cook, whisking constantly, 3 minutes or until thickened. Whisk in dry mustard, next 3 ingredients, and 1/2 cup Cheddar cheese, stirring until cheese is melted. Remove from heat. Season with salt to taste.
5. Gently stir together pasta, cheese sauce, half each of cooked bacon and leeks, and egg yolk. Stir in Gruyère cheese and remaining 1/2 cup Cheddar cheese. Spoon pasta mixture into a buttered 8-inch cast-iron skillet, and sprinkle with remaining bacon and leeks. Drizzle with cream; sprinkle with breadcrumbs.
6. Bake at 375° for 35 minutes or until golden and bubbly. Let stand 15 minutes before serving.