Cook a pound of macaroni.
In a big pot, melt five tablespoons of butter. Add six tablespoons of flour. Whisk well for about a minute. Stir in a teaspoon of mustard powder and a teaspoon of salt.
Gradually whisk in five cups of milk, bring to a boil, then simmer for about five minutes, until thickend. Turn off the heat, and stir in four cups of shredded sharp cheddar.
Add the macaroni to the sauce and cook over medium heat until it's heated through.
Put the mixture into a 13x9 dish and top with bread crumbs. Broil until the bread crumbs are toasted.