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Heat the unsweetened chocolate, confectioners sugar, and heavy cream in a double boiler stirring constantly, over medium heat.
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Remove from the heat when the chocolate has melted.
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Stir in the vanilla extract.
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Place melted chocolate frosting in a bowl, then place into a bigger bowl that has been half way filled with ice water.
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Let the frosting set until cold and thick, then remove from the ice water.
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Beat in the butter on high speed with an electric mixer until creamy.