Put the milk and macaroni in a medium saucepan. Bring the milk to a boil over medium heat, stirring frequently to keep the macaroni from clumping. Continue to boil, stirring frequently, until the macaroni is tender and the milk has thickened to the consistency of heavy cream, 4 to 5 minutes.
Remove the saucepan from the heat, add the Monterey Jack cheese, mozzarella, cream cheese, sour cream, butter, cayenne and 1 1/2 teaspoons salt and stir until smooth, thick and creamy. Fold in the dill and chives. Serve hot. (The dish will thicken as it cools; thin it out with a little hot water if desired.)
For the smoothest and creamiest results, it's worth the extra minute it takes to grate the cheese yourself. The pre-shredded variety often contains cellulous, which can give this mac and cheese a clumpy texture.
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