Boil the macaroni as directed on the package until al dente, then drain and reserve.
In the same pot over medium-low heat, melt the butter and add the milk, whisking until heated through but not simmering.
Whisk in the garlic powder, mustard powder, salt, and pepper until well combined.
Gradually add the shredded cheeses in small handfuls, stirring continuously until each addition is completely melted and the sauce is smooth.
Return the cooked pasta to the pot, fold it into the sauce until every piece is coated, and serve right away.