Pre heat oven to 350 degrees. Butter a large baking dish. Set aside.

Boil pasta in salted water until just al dente. Do not overcook. You want the pasta slightly firm. Drain. Rinse with cold water, set aside.

In a large sauce pan, melt the butter. Sprinkle the butter with flour and whisk on medium low heat until the flour has cooked through. This is what the fancy pants call a roux. Add in the milk, mustard, salt, white pepper and black pepper. Continue cooking on low heat until mixture is thick and creamy- stirring constantly -about 5 minutes.

Beat the egg in a small bowl. Take about one quarter cup of the roux from the pan and add it to the egg to temper. This is a fancy pants way to say- Don't scramble the egg.

Whisk until smooth. Add the egg mixture back into the pan and cook another 1-2 minutes. Add in the all but 1/2 cup of the cheese into the pan and stir with a wooden spoon until the cheese has completely melted.

Off heat, fold in the cooked pasta. Check for seasonings and add more salt or pepper if desired.

Pour into a buttered baking dish.

Sprinkle with the remianing cheese and top with panko bread crumbs.

Bake at 350 degrees for 20-30 minutes until the bread crumbs are golden brown and the cheese is bubbling.