Stir the macaroni, 1 cup of the milk and 1/4 teaspoon salt in a microwave-safe 4-quart bowl. (It is important that you use a bowl large enough to prevent the milk from boiling over.) Cover tightly with plastic wrap; cut a small slit in the center with the tip of a paring knife to vent excess steam. Microwave on high (at 100 percent power) for 4 minutes in either an 1,100- or a 700-watt oven. Uncover (be careful to avoid the hot steam) and stir. Cover and microwave on high (at 100 percent power) until the macaroni is about two-thirds cooked, a little harder than al dente, 1 1/2 minutes in an 1,100-watt oven or 3 1/2 minutes in a 700-watt oven.
Add the remaining cup of milk, American cheese, Jack cheese, Parmesan, cream cheese and mustard. Stir well, cover tightly and microwave on high (at 100 percent power) until all the cheeses have melted, 4 minutes in an 1,100-watt oven or 4 1/2 in a 700-watt oven. Stir until thoroughly combined. If there are still some pieces of unmelted cheese, microwave again, covered, in 30-second increments. (The sauce will continue to thicken as it sits.) If desired, transfer to a decorative dish for serving.
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