In sauce pan, melt butter. Whisk in flour and dry mustard powder, and continue whisking until smooth (2-3 minutes). Add milk, onion, bay leaf, paprika and cayenne pepper and simmer 10 minutes (until thickened). Remove from heat & discard bay leaf. Temper egg (add slowly to dish to avoid it scrambling) and stir in salt, pepper and cheese. Pour over pasta and put in baking dish (2 quarts).
Melt butter. Stir in cheese and corn flake crumbs. Sprinkle evenly over pasta.
Bake at 350 F (180 C) for 30 minutes. Let sit for 5-10 minutes before serving so everything settles.