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Line a large rimmed cookie sheet (12X18-inches) with parchment paper, buttering below and on top of the parchment to help it stick to the pan.
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In a large pot over medium heat, combine the brown sugar, butter, and corn syrup.
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Bring to a boil and cook, stirring constantly, for three minutes.
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Remove from the heat and stir in the sweetened condensed milk.
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Return to medium heat and bring to a boil for an additional three minutes, stirring constantly.
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Stir in the rice krispies (or put the rice krispies in a large bowl and pour the hot mixture over the cereal). Mix to combine.
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Spread the krispie mixture into the prepared pan, pressing lightly.
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Cool completely before cutting into squares.