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Preheat oven to 250 degrees F (95 degrees C).
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Place popcorn in a very large bowl.
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In a medium saucepan over medium heat, melt butter.
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Stir in brown sugar, corn syrup and salt.
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Bring to a boil, stirring constantly.
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Boil without stirring for 4 minutes.
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Remove from heat and stir in baking soda and vanilla.
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Pour in a thin stream over popcorn, stirring to coat.
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Place in two shallow baking pans or dishes and bake in preheated oven, stirring every 15 minutes, for 1 hour.
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If you like chewy popcorn, bake for less time.
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Remove from oven and let cool completely before breaking into pieces.