Lightly butter 15x10x2-inch baking dish. Melt ¼ cup butter in heavy large saucepan over medium heat. Add leeks; stir to coat. Cover saucepan and cook until leeks are tender, stirring occasionally, about 12 minutes (do not brown). Uncover saucepan; add flour. Stir 2 minutes. Add milk; bring to simmer, stirring often. Add cheese, mustard, and pepper sauce. Stir until cheese melts. Remove from heat. Season cheese sauce to taste with salt.

Whisk eggs in medium bowl. Gradually whisk in 1 cup cheese sauce. Stir egg mixture into cheese sauce in saucepan.

Meanwhile, cook pasta in large pot of boiling salted water until just tender but still firm to bite (about 8 minutes), stirring occasionally. Drain. Return to pot. Stir cheese sauce into pasta in pot. Transfer to prepared baking dish. [Can be made 2 hours ahead. Let stand at room temperature.]

Preheat oven to 400°. Bake pasta until cheese sauce is bubbling around edges and some ends of pasta are golden brown, 25 to 30 minutes. Let stand 15 minutes. Serve hot.