In the Instant Pot or electric pressure cooker, combine the pasta, water, butter, mustard powder, salt, and pepper. Stir.
Set it for 4 minutes on high pressure, manual setting. Lock the lid and make sure the vent is closed.
As soon as the 4 minutes is up, release the pressure.
Remove the lid. Add the milk and cheese; stir to blend.
Serve immediately or cover and leave the pot on warm for 30 to 45 minutes.
Alternatively, pour the macaroni and cheese into a casserole. Top with about 1 cup of buttered bread crumbs — 1 cup soft bread crumbs tossed with 2 tablespoons of melted butter — and bake at 375 F until the bread crumbs are browned. Or toast the buttered bread crumbs on a baking sheet separately and sprinkle them over servings.