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Preheat oven to 350 degrees.
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Combine the butter, cocoa and water in a saucepan. Cook, stirring constantly, over low heat until the mixture is smooth and the butter has melted; remove from heat and cool.
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Combine the buttermilk, eggs, baking soda and vanilla; beat on medium speed with a mixer until smooth.
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Combine the butter mixture and buttermilk mixture; beat until well blended.
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Combine the sugar, flour and salt in a bowl; mix well.
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Gradually add the flour mixture to the buttermilk mixture; beat until blended. The batter should be thin.
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Spray three 8" round cake pans with cooking spray and line them with wax paper.
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Pour the batter evenly into the pans and bake at 350 degrees for 22-24 minutes or until cake tests for doneness. Cool in the pans on wire racks for 10 minutes; remove from pans and cool completely.