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Combine 1/4 cup sugar, cornstarch, flour, cocoa and salt in small bowl.
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Blend well.
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Stir in 1/4 cup milk and egg yolks.
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In a saucepan, combine remaining 3/4 cup sugar and remaining 2 cups milk; place over medium heat and gradually bring to a boil.
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Remove from heat.
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Stir 1 cup hot milk mixture into the cocoa mixture, then return to saucepan.
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Bring to a boil over medium-high heat, stirring constantly.
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Let mixture boil for 5 minutes while continuing to stir.
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Remove from heat and stir in butter and vanilla.
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Pour into pie shell and let chill 2-3 hours before serving.
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Top each slice with a dollop of Cool Whip when serving.