FOR THE PASTA: Put the macaroni, water, butter, and salt in the Instant Pot and pressure cook for 4 minutes.
Grate most of the cheese and dice the remaining cheese into cubes. Heat the milk without boiling.
FOR THE SAUCE: Melt butter in the Instant Pot, add flour, mustard, and whisk until roux forms. Add hot milk and whisk until thickened. Add salt, pepper, sugar, grated cheese, and stir until melted.
Add in cubed cheese and pasta. Optionally, heat slightly without boiling.
FOR THE TOPPING: Mix crumbs, melted butter, and Parmesan cheese. Spread over the pasta and cheese sauce then bake at 375 degrees for 10-20 minutes until golden brown.
