Line a 9-by-13-inch baking dish with parchment, leaving a 2-inch overhang on at least 2 sides.
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 Cheddar, mozzarella, cream cheese, pimentos, sour cream, hot sauce, cayenne, 1 teaspoon salt and a few grinds of black pepper and stir until smooth, thick and creamy. Spread into the prepared baking dish and freeze until firm, about 30 minutes.
Use the parchment overhang to lift the macaroni in a block from the dish. Cut into 2-inch squares, transfer to a baking sheet and freeze until completely firm, about 30 minutes.
Meanwhile, fill a large, heavy-bottomed pot with 2 inches of oil and heat until a deep-frying thermometer registers 375 degrees F. Fry the mac and cheese squares in batches until golden brown on the outside and warm and gooey on the inside, 2 to 3 minutes per batch. Transfer to a paper towel-lined plate. Serve warm with more hot sauce drizzled over top.
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