In a pot over medium heat, bring salted water to a boil. Add macaroni and cook about 1 minute shy of the package directions or until slightly firm. Drain well and toss with one tablespoon butter. Set aside.
In a bowl, combine butter, crackers, and Parmesan cheese. Set aside.
In a saucepan over medium heat, add the 1/2 cup butter and cook until melted. Add flour and whisk until smooth. Cook, stirring constantly, for about 2 to 3 minutes or until lightly browned and slightly nutty in aroma.
Gradually add milk and heavy cream, whisking vigorously to prevent lumps. Bring to a simmer, whisking regularly.
Add ground mustard, nutmeg, cayenne pepper, salt, and pepper. Reduce heat and continue to simmer, whisking occasionally, for about 2 to 3 minutes until the mixture thickens and coats the back of the spoon.
Add cheese in three parts, whisking until cheese is melted before adding the next batch.
In a large bowl, combine cheese sauce and cooked macaroni. Transfer into a lightly greased baking dish.
Sprinkle top with the buttered crackers.
Bake in a 350 F oven for about 15 to 20 minutes or until the top is lightly browned and the mixture is bubbly.