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Place the milk in a large, heavy saucepan over medium-high heat.
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Bring to a boil and allow skin to form on top.
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Do not stir.
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The skin will thicken and puff up.
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Reduce heat to medium and remove the skin.
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Increase heat to medium-high and repeat the process 3 times.
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Add the sugar and vanilla bean, and stir until the sugar has melted, about 2 minutes.
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Reduce heat to low and stir in the baking soda.
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Simmer milk, stirring often, but taking care not to break the film of cooked milk forming, on the inside of the pan.
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Simmer, skimming the foam (not skin) until reduced by half, about 1 hour.
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Reduce the heat to very low and simmer for another 3 and 1/2 hours, stirring and skimming foam occasionally.
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The mixture will brown as it cooks.
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Strain the completely cooked mixture through a sieve into a bowl, cover and refrigerate.