Baked Penne With Farmhouse Cheddar and Leeks

Lightly butter Emile Henry oval Gratin baking dish. Melt 1/4 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, stirring occasionally. Drain. Return to pot. Stir cheese sauce into pasta in pot. Transfer to prepared baking dish. DO AHEAD Can be made 2 hours ahead. Let stand at room temperature. Preheat oven to 400°F. 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. Unlike classic mac and cheese, which is thickened by a roux (a flour and butter mixture), this one uses eggs to make a rich, gooey custard. But be careful when adding the hot cheese sauce to the eggs; you’ll need to whisk the sauce in slowly so that the eggs don’t curdle.