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In a large saucepan, bring milk to a boil over medium-high heat.
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Remove from heat and strain through cheesecloth.
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Return to pan.
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Cut vanilla bean in half and pour the seeds in the milk.
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Stir in the sugar and replace the pan on medium-high heat, stirring constantly until the sugar is dissolved.
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Just as the milk mixture begins to boil, stir in the baking soda.
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Reduce the heat to medium, stirring constantly until mixture thickens. When a wooden spoon drawn through the mixture leaves the bottom of the pan visible, and the mixture is light brown in color, remove the pan from the heat.
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Place the pan in an ice bath and stir constantly until dulce de leche is cold.
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Store in airtight container in refrigerator.
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Pour into sterile jars, and store in the refrigerator.