Heat the oven to 375°F and butter a 9-by-13-inch baking dish with 1 tablespoon butter.
Boil the macaroni in salted water for 2 minutes less than the package direction, then drain it well.
Melt 2 tablespoons butter in a saucepan over medium heat, whisk in the flour and cook for 1 minute.
Whisk in the warm milk gradually and simmer for 4 to 6 minutes until the sauce coats a spoon.
Remove the pan from the heat, whisk in the mustard, garlic powder and pepper, then stir in the cheddar a handful at a time until smooth.
Fold in the macaroni, transfer it to the baking dish and level the top without packing it down.
Melt the remaining tablespoon butter, mix it with the cracker crumbs and scatter them evenly over the macaroni.
Bake for 20 to 25 minutes until the edges bubble and the topping is golden, then rest for 10 minutes before serving.
